An interpretation of the polarized crystal spectra of mixed metal trimethylammoniumchlorometallate
salts
by Martha Jean Lindbeck
A thesis submitted in partial fulfillment of the requirements for the degree of DOCTOR OF
PHILOSOPHY in Chemistry
Montana State University
© Copyright by Martha Jean Lindbeck (1983)
Abstract:
Crystals fitting the formula (formula not captured in OCR)where M and M' represent the divalent ions
of the first row transition metals Mn, Fe, Co, Ni and Cu have been found to be striking examples of
pleochroism.
Because these highly colored crystals possess orthorhombic symmetry, their unit cell axes are easily
oriented with respect to the electric vector in polarized light. Through observations of the polarized
crystal spectra coupled with group theoretical calculations, energy level diagrams have been
constructed for the various ions in (formula not captured in OCR) symmetry. In order to explain all the
observed peaks transition moment integrals for both electric and magnetic dipole transitions had to be
considered. In addition to the spectral work cell parameters for the previously unpublished pure Fe and
Ni analogs have been determined using a Supper precession camera. Analyses of the mixed crystals for
individual metal content established the relative ease with which these ions were incorporated into the
lattice site. AN INTERPRETATION OF THE POLARIZED CRYSTAL SPECTRA OF MIXED
METAL TRlMETHYLAMMONIUMCHLOROMETALLATE SALTS
by
MARTHA JEAN LINDBECK
A thesis submitted in p a r tia l fu lfillm e n t
o f the requirements fo r the degree
Of
DOCTOR OF PHILOSOPHY
in
Chemistry
MONTANA STATE UNIVERSITY
. Bozeman, Montana
March, 1983
D 37g
L M _
C o p .^
11
APPROVAL
o f a thesis submitted by
Martha Jean Lindbeck
This thesis has been read by each member o f the thesis committee
and has been found to be s a tis fa c to ry regarding content, English usage,
form at, c ita tio n s , b ibliographic s ty le , and consistency, and is ready
fo r submission to the College of Graduate Studies.
Date
Chairperson, Graduate Committee
Approved fo r the Major Department
IAdr.
^8
__
IW
CgA7 //
Head, Major Department
Date
Approved fo r the College o f Graduate Studies
"3 Date
/ - f 2
y y
*
Graduate Dean
iii
STATEMENT OF PERMISSION TO USE
In presenting th is thesis in p a r tia l fu lfillm e n t o f the req u ire­
ments fo r a doctoral degree a t Montana S tate U n iv e rs ity , I agree th at
the Library shall make i t a v a ila b le to borrowers under the rules o f the
L ib rary .
I fu rth e r agree th a t copying o f th is thesis is allowable on­
ly fo r scholarly purposes, consistent w ith " f a i r use" as prescribed in
the U.S. Copyright Law.
Requests fo r extensive copying or reproduc­
tio n o f th is thesis should be re fe rre d to U niversity M icrofilm s In te r ­
n a tio n a l, 300 North Zeeb Road, Ann Arbor, Michigan 48106, to whom I
have granted "the exclusive rig h t to reproduce and d is trib u te copies of
the d is s e rta tio n in and from m icrofilm and the r ig h t to reproduce and
d is trib u te by abstract in any fo rm a t."
Signature
Date
a3
CtIanUia. J.
_____
H y s ____________
V
TABLE OF CONTENTS
Page
L is t o f Tables..........................
v ii
L is t o f Figures........................
v iii
A bstract..........................................................................
x
INTRODUCTION.................................
I
EXPERIMENTAL METHODS..............................
11
Syntheses...................'........................................ .— .................................
11
Analyses....... ..........................................................
14
Morphology and S tru c tu re .......................................................................
22
Spectra..................
28
DISCUSSION..............................................................................
43
Manganese Spectra.....................................................................................
86
Cobalt Spectra...................................................
94
Iron Spectra...............................................................................................
105
Nickel Spectra..................
109
Copper Spectra................
113
CONCLUSION................................................................
122
LITERATURE CITED....................................
124
vi
LIST OF TABLES
TABLE
I.
II.
Page
Cell Dimensions and Space Group fo r TMAMCl3 -EH2OS a lt s .. .
6
Inform ation on Metals o f In te re s t fo r the Series
TMAMxM ^ xC l 3 -EH2O.....................................
9
Chloride and Metal Content o f the Series TMAMCi 3 =EH2O------
15
Mole Fractions in Cu Containing S a lts ................
EO
V.
Mole Fractions in Mn Containing S a lt s .......................... ............
EO
V I.
Cell Dimensions fo r Members o f the Series TMAMCl3 =EH2O ...
E6
III.
IV .
V II.
Theoretical Values o f b. in Square Planar Arrangement fo r
TMAMCl3 =EH2O in Pnma............................. .......................... ..................
11
V III.
C ell Dimensions o f Some C rystals in the Mn-Cu S ub series..
53
IX .
Cell Dimensions o f Some Crystals in the Mn-Co S ubseries..
53
X.
Colors Exhibited by TMACoxM ^ xC l3=EH2O C ry s ta ls ....................
66
X I.
X II.
Colors Exhibited by TMAMnxMj xC l 3 =EH2O C ry s ta ls ..................
. 70
Russell-Saunders Terms fo r Free Io n s ..............................
76
X III.
S p littin g o f R-S Terms in Various Symmetries..........................
78
XIV.
Ground States in Various Symmetries................ '......... ................
79
XV.
Representations o f the Dipole Moment Operator...................
80
XVI.
Normal V ib ratio n M odes.................................................................
83
Location and P o la riza tio n o f M n (II) Peaks............ ...................
9E
X V II.
vi i
LIST OF FIGURES
FIGURE
Page
1.
The Two Metal S ites in TMAgMgCl^................................
4
2.
U nit Cell o f T M A M n C l g i n the b-c Plane...........................
7
3.
Some Habits Displayed by the Series TMAMxM ^ xClgeEHgO-----
23
4.
Location o f C rystallographic Axes and P o la riza tio n
D ire c tio n s ...............................................................
28
5.
Polarized Crystal Spectra o f TMAMnClgeEHgO........................
31
6.
Crystal Spectrum o f TMACoClgeEHgO in x- or y;-P o la r ized
L ig h t...........................................
7.
Crystal Spectrum o f TMACoClgeEHgO in z-P o larized L ig h t ...
8.
Polarized Crystal Spectra o f TMAMnxC o ^ xClgeEHgO (V is ib le
Region)..............................................................
9.
32
33
34
P olarized Crystal Spectra o f TMAMnxC o ^ xC lg»EHgO or
TMACoClgeEHgO (IR Region)................................................................
35
10.
P olarized Crystal Spectra of TMAMnxF e ^ xClg-EHgO ( I R ) ------
36
11.
P olarized Crystal Spectra o f TMAMnxN i ^ xClgeEHgO (V is ib le
Region)................
37
12.
Polarized Crystal Spectra o f TMAMnxN i ^ xClg-EHgO( I R ) ; . . .
38
13.
Polarized Crystal Spectra o f TMAMnxC u ^ xC lg*EHgO (V is ib le
Region)........................
14.
39
Polarized Crystal Spectra o f TMAMnxC u ^ xC lg»EHgO (IR .
Region)................................... - ...............................................................
40
15.
Unpolarized Crystal Spectrum o f TMAMnxFe^ xClg-EHgO...........
4.1
16.
Unpolarized Crystal Spectrum o f TMACoClg-EHgO......................
42
17.
Mole Fraction Plots o f TMAMnM1ClgeEHgO......................................
45
vi I i
FIGURE
,
'
Page
18.
Mole Fraction Plots o f TMACoM'C l 3 »
19.
M o le F ra c tio n p io ts o fT M A C u M 1C l 3 ^ H 2O . . . . ........ ....................
47
20.
Packing Hole Formed by Four C hlorides....................
50
21.
T e tra g o n a l, In d ic a tric e s .......................................
22.
Double Refraction o f Iceland Spar................
57
23.
Ray D irections and Wave Normals....................................................
59
24.
B ia x ia l In d ic a t r ix ...................................................
61
25.
U nit Cell Axes and Bonding D ire c tio n s .......................................
64
26.
Colors Observed in Co-Ni C rystals in Polarized L ig h t........
68
27.
S p littin g o f the d O rb itals in Various Symmetries...............
73
28.
C2 Rotation in Normal Coordinates................................................
82
29.
M n (II) Spectral T ra n s itio n s ...................
89
30.
Possible Energy Level Diagram fo r C o ( I I ) .................................
98
31.
P referred Energy Level Diagram fo r C o ( I I ) ...............................
101
32.
Possible Energy Level Diagrams fo r F e ( I I ) ................
106
33.
Energy Level Diagram fo r N i ( I I ) ................
HO
34.
S p littin g o f the d O rb itals in D2h............................. ........... . .
116
35.
Energy Level Diagram fo r A2CuCl^ in D2h............................
117
36.
Energy Level Diagram fo r TMACuC13 *2H20 in D2h. .....................
117
......................
46
.56
ix
ABSTRACT
C rystals f i t t i n g the formula [(CH 3 ) 3NH]MxMj_xC l 3 «2H20 where M and
M1 represent the d iv a le n t ions o f the f i r s t row tra n s itio n metals Mn,
Fe, Co, Ni and Cu have been found to be s trik in g examples o f pleochroism.
Because these highly colored crystals possess orthorhombic
symmetry, t h e ir u n it c e ll axes are e a s ily oriented w ith respect to the
e le c tr ic vector in po larized lig h t .
Through observations of the pola­
rize d crystal spectra coupled w ith group th e o re tic a l c a lc u la tio n s ,
energy level diagrams have been constructed fo r the various ions in D_.
symmetry.
In order to explain a ll the observed peaks tra n s itio n
moment in te g ra ls fo r both e le c tr ic and magnetic dipole tra n s itio n s had
to be considered.
In addition to the spectral work c e ll parameters
fo r the previously unpublished pure Fe and Ni analogs have been deter­
mined using a Supper precession camera.
Analyses o f the mixed crys­
ta ls fo r in d ivid u al metal content established the r e la tiv e ease with
which these ions were incorporated in to the la t t ic e s it e .
I
INTRODUCTION
The h is to ry o f a lk y lammonium chlo ro m etallates, as i t appears in the
s c ie n tific lit e r a t u r e , dates as fa r back as 1908 a t which time Groth ( I )
published his work on the c o p p e r(II) s a lt [(CH 3 ) 3NH]CuC13 -2H20.
This
is one member, although a hydrated one, of a series o f compounds
represented by the general formula:
AmnMXn t2
where Am = an ammonium s a lt
M = a metal
X = a halogen
n = an in teg er such th a t I < n & 4
In 1933 Reniy and Laves (2 ) prepared a large number o f these s a lts using
various a lk y l amine hydrochlorides and c o p p e r(II) chloride as s ta rtin g
m a te ria ls .
For the most part t h e ir work, lik e Groth's , was prepara­
tiv e and d e s c rip tiv e .
Except fo r the preceding papers, there is a
dearth o f lit e r a t u r e concerning th is series u n til the 1960's.
Research
in te re s t in these s a lts was renewed a t th is time fo r a number o f rea­
sons.
Instrumental technology had caught up with theory in the areas
o f magnetics and spectroscopy, and the structures o f these compounds be­
came the focus o f increased in te re s t.
Good crys ta ls o f these salts
were e a s ily prepared, and thus the series provided a convenient source
o f m aterial fo r te s tin g e x is tin g th eo ries.
In recent years there has been considerable in te re s t in compounds
which contain metals with unpaired d electrons and which are composed
.
o f lin e a r chains and layers held together p rim a rily by hydrogen bonding
and van der Waals forces.
Some o f these serve as e x c e lle n t experimental
2
models o f one- and two-dimensional magnetic systems;
low temperature
heat capacity and s u s c e p tib ility data have been obtained on such com­
pounds and f i t t e d to various th e o re tic a l models.
As experimental data accumulated on th is s e rie s , i t was discovered
th a t some members displayed a property known as thermochromism.
In
1968 Day (3 ) published a paper in which he defined thermochromism as
"the re v e rs ib le change in color o f a compound when i t is heated or
cooled" and went on to review the e x is tin g lit e r a t u r e on such compounds.
Thermochromism can be c la s s ifie d as e ith e r continuous or discontinuous.
In the former the color change is gradual and is due to the temperature
dependence o f the lin e widths of the absorption bands.
In discontin­
uous thermochromism, which is more in te re s tin g chem ically, there is a
sharp color change corresponding to some c h a ra c te ris tic temperature.
This is due to a change in coordination geometry or ligand conn ectivity
which re su lts in an energy s h if t o f the v is ib le absorption bands.
Remy
and Laves (3) had noted th a t some o f t h e ir c o p p e r(II) s a lts were th e rmochromic,. but beginning in 1964, W ille t t was among the f i r s t to explore
the underlying causes o f these observed color changes.
. In 1974 he pub­
lish ed a paper (4 ) in which he described two members o f the follow ing
series which were e x c e lle n t examples o f discontinuous thermochromism:
Am2CuCl4
where Am = (CH3 ) 2CHNHg+ or IPA
= (C2H5 ) 2NH2 1- or DEA
Crystals o f both o f these s a lts are green a t room temperature and upon
heating undergo an abrupt color change to yellow ;
a t 43°C and the IPA2CuCl4 changes a t 56°C. .
the DEA2CuCl4 changes
D iffe re n tia l thermal
3
analysis data and near IR spectra above and below the tra n s itio n temper­
ature suggest a change in coordination geometry o f the CuCl4- species.
Comparison o f these spectra with those o f known structures indicated a
square planar geometry fo r the low temperature stru ctu re and a fla tte n e d
tetrah ed ral geometry fo r the high temperature s tru c tu re .
This work has
recently been extended to the IPACuClg s a lt (5) which changes from
brown to orange a t S l 0C.
The corresponding s tru c tu ra l s h if t is from
bibridged lin e a r chains o f the dimer CUgClg- to trib rid g e d chains of
(CuClg)JI".
In conjunction with the in te re s t in the magnetics o f these
systems, EPR and s u s c e p tib ility data were collected along w ith the
s tru c tu ra l inform ation.
In te re s t in the geometry, stru ctu re and re la te d properties o f the
s a lts in th is p a rtic u la r series led to the synthesis o f a wide v a rie ty
o f c ry s ta ls .
In the course o f these in vestig atio n s some s im ila r com­
pounds were discovered which held a s lig h tly d iffe r e n t stoichiom etry than
the one previously c ite d .
One such class o f compounds which received
a tte n tio n is characterized by the formula:
TMA3M2C l 7
where TMA = t r i methyl ammonium, (CH3 ) 3NH+
Members o f th is series fo r which M = Mn and Cu have been prepared and
studied ( 6 ,7 ) .
In th is stru ctu re the d iv a le n t metal ions e x is t in two
d iffe r e n t environments;
one is characterized by chains o f face-sharing
MClg octahedra, the other by discrete MCl4- tetrahed ra.
tu ra l arrangement is illu s t r a te d in Figure I .
This struc­
4
Figure I
The Two Metal S ites in TMA3M2Cly
The existence o f two d iffe r e n t s ite s in such a system leads quite
lo g ic a lly to an attempt to grow s a lts with a d iffe r e n t metal in each o f
the s ite s .
M n (II) and C u (II) w ill re a d ily adopt e ith e r coordination,
but other f i r s t row tra n s itio n metals are not so f le x ib le .
For example,
C o (II) and Z n ( II) p re fe r the tetrahed ral geometry in MCl4" whereas
cations such as N i ( I I ) and C d (II) form the octahedral chains more read­
ily .
Clay, e t. al_., (7) attempted to grow the mixed metal s a lt with
the formula TMA3CoCuCly where C o (II) was expected to occupy the te t r a ­
hedral s ite .
Continuous s o lid solutions were obtained ra th e r than a
single product with the desired stoichiom etry.
Other than th is paper,
nothing fu rth e r appears in the lit e r a t u r e in reference to mixed metal
5
s a lts o f th is p a rtic u la r series.
The existence o f two d iffe r e n tly coordinated metal s ite s w ithin a
sing le stru ctu re is a ra th e r unusual occurrence.
The question arises
as to whether, under the appropriate conditions, various metals could
be forced to assume t h e ir preferred geometries in such a system.
This
research began as an e f f o r t to prepare such c rystals with the stoichiom­
e try TMAgMM1Cl^ where trim ethyl amine hydrochloride and the s ix possible
pairs from the d iv a le n t chlorides of Mn, Co, Ni and Cu were used as
s ta rtin g m a te ria l.
When attempts to grow these mixed metal s a lts from
stoichiom etric amounts in anhydrous solutions o f methanol and ethanol
proved unsuccessful, the syntheses were performed in aqueous solution.
Q u a lita tiv e analyses of the crys ta ls obtained from the l a t t e r procedure
indicated th a t both metals were present in each p a ir.
However, quanti­
ta tiv e analyses along with precession photographs of the Mn-Co analog
subsequently showed i t to be isomorphic with the compound TMAMnClg-EHgO.
A mixed metal product had indeed been prepared, but not one with the
desired tw o -s ite .s tru c tu re .
The crys ta ls which had been grown had the same stoichiom etry as the
C u (II) s a lt studied by Groth in 1908 ( I ) ;
these are the hydrated mem*
bers o f the series f i r s t prepared and studied by Remy and Laves (2 ) .
A lit e r a t u r e search revealed th a t fo r th is hydrated series crystals con­
ta in in g Mn, Co and Cu had been studied in d e t a il, again in connection
with th e ir s tru c tu ra l arid magnetic p ro p erties.
s a lt was prepared f i r s t ;
H is to r ic a lly , the Cu
i t s crystal stru ctu re was the f i r s t to be e lu ­
cidated and there is the most inform ation on th is compound in the l i t ­
eratu re ( 8 ) .
Heat capacity and s u s c e p tib ility measurements have also
6
been made on both the Cu and Co analogs ( 8 ,9 ) ;
only s tru c tu ra l work has
been done on the Mn member o f th is series (1 0 ).
Of these three s a lts ,
the Mn and Co analogs are isomorphic;
to the Pnma space group.
they are orthorhombic and belong
The Cu analog is monoclinic as Groth o rig in ­
a l l y reported, and it s space group is PZ1Zc.
Cell dimensions and the
space group fo r these compounds are reported in Table I .
Table I
Cell Dimensions and Space Group fo r TMAMClg-ZHgO S alts
O .
a.(A)
O
b (A)
O
c (A)
TMAMnCl3 -ZH2O
16.779(3)
7.434(1)
8.Z27(1)
Pnma
TMACoCl3-ZHgO
16.671(3)
7.Z73(1)
8 . 113(Z)
. Pnma
7.864(11)
16.730(Z3)
TMACuCl3-ZHgO
7.479(10)
Group
PZ1Zc
The names o f these s a lts are in d ic a tiv e o f th e ir s tru c tu re ;
fo r
example, the manganese analog is named trim eth yl ammonium ca ten a-d i-p Chlorodiaquomanganese(II) c h lo rid e.
It s stru ctu re is composed o f in ­
f i n i t e chains o f edge-sharing MnCl4 (HgO) 2 octahedra which run p a ra lle l
to the b^ axis with bridging Cl atoms and O atoms in the trans positions.
The chains are hydrogen bonded along the .c axis through the fre e Cl™
anions;
the trimethylammonium cations l i e between the planes o f hydro­
gen bonded chains.
th is ;
The cobalt stru ctu re is completely analogous to
the only differences occur in the c e ll dimensions due to the
s lig h tly sm aller crystal io n ic radius o f the d iv a le n t cobalt.
Figure Z on the follow ing page is a view along the a_ axis o f the
u n it c e ll o f the TMAMnCl3-ZHgO s a lt .
The chains l i e along the
7
Z
Figure 2
N
V-
U nit Cell o f TMAMnCl3 -ZH2O in the b-c Plane
8
c rys ta llo g rap h ic axis and are hydrogen bonded in the £ d ire c tio n .
In the c o p p e r(II) s a lt d is to rtio n o f the CuCl4 (H2O) 2 octahedra re ­
sults in a s lig h tly d iffe r e n t packing arrangement which is s u ffic ie n t to
force th is analog o f the series into a d iffe r e n t space group.
By con­
ventional choice o f axes in the monoclinic P2^/c group* the chains now
run p a ra lle l to a ra th e r than jb as in the orthorhombic c e ll.
the bonding patterns remain the same.
However,
Two o f the coordinated Cl atoms
are found a t a s lig h tly g reater distance from the central Cu atom than
are the remaining four atoms o f the coordination sphere.
Although one
angle is now d isto rte d from 9 0 °, the chains remain edge-shared with
hydrogen bonding and van der. Waals forces s t i l l in e ffe c t.
A ll o f the work done on the trim e th y lammoniurn chlorom etallate ser­
ies has been w ith these three pure metal compounds.
A look a t the per­
io d ic ta b le raises the question, can the pattern be completed?
That is
to say, can the F e ( I I ) , N i ( I I ) and Z n ( II) analogs be prepared, and w ill
they adopt one of the above structures?
Table I I li s t s the metal atoms
o f in te re s t along with t h e ir electron configurations and crystal io nic
r a d ii.
The values o f the ra d ii are those reported by Shannon and
P re w itt (1 1 ).
Synthesis o f the series TMAMxM ^ xC l 3 -ZH2O, although inadvertent,
yield ed some in trig u in g observations on the chemical and o p tical proper­
tie s o f the pairs as compared to the pure metal end members.
Since the
mixed metal c rystals in th is series were e a s ily grown, i t was deemed
worthwhile to pursue the hydrated s in g le -s ite metal p a ir system in some
d e ta il.
There are several aspects o f th is series which m erit a tte n tio n .
9
Table I I
Information on Metals of. In te re s t fo r the Series TMAMxM|_x C13 -2H20
column a
Metal
a_
Jb
M n (II)
3d5
0.82
Pnma
F e ( II)
3d6
0.78
Pnma*
C o (II)
3d7
0.74
Pnma
N i(II)
3d8
0.70
Pnma*
C u (II)
3d9
0.73
P21/c
Z n ( II)
3d1 0 ‘
0.75
?
C d (II)
4d 10
0.95
?
C
column b
outer shell configuration
O
crystal io n ic r a d ii (A) fo r high spin six coordinate complex
column c
space group, a s te ris k s ig n ifie s compound not reported in the
lit e r a t u r e
Composition and stru ctu re are two o f the areas which were investigated
in th is work.
These properties are o f p a rtic u la r in te re s t in the Cu
containing c rystals since, the pure Cu analog has a d iffe r e n t structure
than the other members o f the s e rie s .
Examination o f the Cu subseries
to ascertain whether these compounds c r y s ta lliz e as the monoclinic P 2 j/c ,
orthorhombic Pnma or perhaps some new stru ctu re was one o f the goals o f
th is research.
C rystals o f the Fe and Ni members o f th is series were
also synthesized and t h e ir structures determined to see i f they followed
the e x is tin g trend indicated by the Mn and Co s a lts .
10
These compounds also e x h ib it a phenomenon known as dichroism, a
property o f c rystals which is discussed a t length in a la t e r section.
B r ie fly ,
they display two d iffe r e n t colors depending on th e ir spatial
o rie n ta tio n .
This dichroism is not to be confused with the discontin­
uous thermochromism mentioned e a r lie r .
The color change observed in
the la t t e r usually involves a s tru c tu ra l s h if t and is temperature depen­
dent while the colors in the former are a re s u lt o f the o p tic al proper­
tie s o f the crystal and are both v is ib le a t room temperature.
The
presence o f two metals seems to enhance th is e f f e c t , as the mixed metal
s a lts are much more strongly dichroic than are the pure metal end mem­
bers o f th is s e ries .
For dichroism to be observable in th is chlorom etallate s e rie s ,
absorption bands must be present in the v is ib le region o f the e le c tro ­
magnetic spectrum.
In conjunction with the o p tical properties manifes­
ted by these c ry s ta ls , spectral data were collected in both polarized
and unpblarized lig h t in order to gain a greater understanding o f what
governing facto rs control the colors displayed by the various mixed
metal s a lts .
The question as to whether metal-metal in teractio n s could
occur in the mixed metal compounds, thus giving ris e to the s trik in g d i­
chroism o f th e s e .c ry s ta ls , was addressed and a careful comparison o f the
r e la tiv e in te n s itie s and locations o f absorption bands in various mem­
bers o f the series was made.
11
EXPERIMENTAL METHODS
SYNTHESES
C rystals o f s a lts f i t t i n g the formula TMAMXMJ_XC13 - ZH2O were grown
fo r eigh t o f the ten possible combinations o f the f i r s t row tra n s itio n
elements from Mn to Cu, in c lu s iv e .
The pure end members, x = 0 , were
prepared by dissolving equimolar q u a n titie s o f the d iv a le n t metal ch lo r­
ides and trim ethyl amineihydrochloride in a minimal amount o f water and
allowing the aqueous solution to slowly evaporate.
The c rystals which
contained metal pairs were prepared by mixing varying mole fractio n s
from x = 0 .2 to % = 0 .8 o f the m etal( I I )
chlorides with a stoichiom etric
amount of the TMA solution and again merely le t t in g the solution evap­
orate slow ly.
I t is c h a ra c te ris tic o f t;he mixed metal series th a t the crystals do
not grow with the same stoichiom etry as th a t o f the o rig in a l solu tio n .
I f a crystal such as TMAMng 5CUq ^Cl3-ZH2O is desired, the s ta rtin g
solution must have a x^n soi n = 0.65 in order to produce a s o lid with
XMn x ta l " 0 -5 0.
The mole fra c tio n found in the crystal vs^ th a t found
in the s ta rtin g solution w ill be tabulated fo r the individual series and
discussed in a la t e r section.
P e tri dishes o f various sizes and surface areas were u t iliz e d ;
practice with the ra te o f evaporation even tu ally led to the optimum
methods o f obtaining crystals o f the desired size and q u a lity .
Even
a f t e r much p ra c tic e , though, there oftentim es seemed to be as much luck
as s k ill involved in the process.
As these s a lts are extremely soluble in w ater, the solutions were
sometimes q u ite viscous before crystals began to grow.
This s o lu b ility
12
made i t impossible to obtain crystals o f th is series containing only Zn
which has a f i l l e d 3d subshell or calcium, with an empty 3d subshell.
Attempts to grow the pure Cd analog y ie ld ed the anhydrous s a lt
TMACdCl3.
Addition o f excess chloride in the form o f concentrated HCl to the
solutions o f the pure Co and Co-Ni mixtures resulted in the formation
o f la rg e r crystals in these systems in a shorter period o f tim e.
This
same e ffe c t was noted as a general ru le fo r the other s a lts as w e ll.
No special precautions were taken in the syntheses o f the Mn, Co,
Ni or Cu containing s a lts , but in the case o f the F e ( II) s a lt and any
mixture in the subseries containing th is m e ta l, a ir oxidation proved to
be a problem.
Solutions o f i r o n ( I I ) chloride were prepared from the
hydrate, the anhydrous s a lt and from iron w ire dissolved in concentra­
ted HCl.
Because i r o n ( I I ) is not very stable in a i r , there is the
p o s s ib ility th a t some i r o n ( I I I ) is present in solutions made from the
i r o n ( I I ) s a lts .
For th is reason the most r e lia b le syntheses o f the Fe
subseries were those done using the solutions o f F e ( II) prepared from
the iron w ire .
In order to obtain good q u a lity crystals o f the pure
Fe analog and it s mixed metal subseries, the s ta rtin g solutions were
placed in a vacuum desiccator containing concentrated HgSO^ as the
desiccant and an in e r t nitrogen atmosphere.
A successful synthesis of
the Fe-Cu mixed s a lt w ith the proper stoichiom etry was never accom­
plished, presumably because o f the high p ro b a b ility o f a redox reaction
occurring between these two metals when both are present in the +2 ox­
id atio n s ta te .
The Fe-Co analog was also not prepared.
The crystals
containing iron were collected and stored under a nitrogen atmosphere.
13
Upon exposure to a i r the pure Fe end member began to show signs o f de
te rio ra tio n in about a week, undergoing a color change from very pale
yellow /green to amber and beginning to look wet.
14
. ANALYSES
Once s u ita b le c rystals o f varying mole fra c tio n had been prepared,
the next step undertaken involved determination o f the actual s to ic h i­
ometry.
Q u a lita tiv e analyses o f each batch indicated th a t both metals
were present;
q u a n tita tiv e measurements were then necessary to deter­
mine whether the mole fra c tio n s found in the crystal showed any r e la ­
tionship to th a t in the solution from which i t was grown.
Because
only a lim ite d amount o f these crys ta ls was a v a ila b le , under fiv e grams
in most cases, a ll samples were weighed on an M5 microbalance and a ll
tit r a t io n s were performed with a 1 . 0 - or 2 .5 -ml microburet.
The chloride content was determined by Fajan's method (1 2 ), i . e .
t it r a t io n o f Cl" w ith AgNO3 using d ich io ro fIu o re sc e in , DCF", as the
in d ic a to r.
The AgCl p re c ip ita te ex is ts as a white c o llo id and adsorp­
tio n o f the in d ic a to r anion, DCF", a t the endpoint causes a color
change in the p re c ip ita te from white to pink.
In d ilu te solutions the
AgCl w ill coagulate as w ell as change c o lo r.
The DCF" in solution is
yellow-green and th is may somewhat mask the white c o llo id a l AgCl p rio r
to the endpoint.
In order to keep the suspension highly dispersed,
and thus the color change more v is ib le , d extrin is sometimes added.
I f the analysis is done a t the micro le v e l, th is work in dicated th a t
the AgNO3 t i t r a n t should be standardized with and without the dextrin
present to make c e rta in th a t th is reagent does not in te r fe r e .
The chloride analyses were used to characterize the pure metal
s a lts and to estab lish th a t th e ir stoichiom etry and th a t o f the mixed
metal s a lts as well were indeed c o rre c t.
Table I I I lis t s ! t h e %C1
found and compares these values to those calculated fo r the pure
15
compounds o f the series TMAMxM ^ xC l 3-ZH2O.
In the e a rly stages o f
th is work c h a rac te riza tio n o f the mixed metal compounds by th e ir chlo r­
ide content was considered, since these analyses can be performed with a
r e la tiv e precision o f two or three parts per thousand, ppt.
However,
i t was subsequently determined th a t th is system is a c tu a lly a s o lid so­
lu tio n whose composition can be contro lled only w ith in a c e rta in range.
The growth o f any one batch o f c rystals with a given stoichiom etry has
an inherent e rro r of as much as 50 ppt r e la tiv e in it s metal composition
The magnitude o f th is v a ria tio n w ith in the c rystals grown from one syn­
thesis makes i t impossible to u t i l i z e the small changes which occur in
the %C1 determination to c a lcu late an accurate weighted average o f the
two metals present.
Since the mole fra c tio n of the metals present in
these crystals was o f p a r tic u la r in te r e s t, other a n a ly tic a l techniques
had to be considered.
Table I I I
Chloride and Metal Content o f the Series TMAMCK-2Ho0
M
%C1 Found
%C1 Calculated
%M Found
%M Calculated
Molar
Ratio
Mn
41.50
41.32
Z1.46
Z1.34
2.997
Fe
'
41.17
----- '
Z1.6Z
—-
Z2.54
40.68
Co
Ni
40. Zl
4 0 .7Z
ZlJl
22.48
3.059
Cu
40.53
39.98
Z4.41
23.89
2.976
16
- Included in Table I I I are the molar ra tio s o f chlo ride to metal fo r
each o f the pure compounds analyzed.
These values are q u ite close to
the expected value o f 3.000 which indicates th a t the c rys ta ls were ac­
tu a lly the desired stru ctu re and stoichiom etry.
The chloride content
was not determined fo r the Fe and Co members o f th is series since X-ray
data showed these c rystals to also be the orthorhombic Pnma stru ctu re.
Because o f the wide range o f mole frac tio n s involved in.any one
metal p a ir subseries, methods o f analyses had to be developed not only to
determine the metals in question, but also to determine them in the pres­
ence o f one another in a single sampleJ
I t is possible to determine a ll
o f the metals used in th is s e rie s , Mn, Fe, Co, Ni and Zn, as well as Cu, .
complexom etricalIy u t iliz in g . EDTA as the t i t r a h t (1 3 ).
EDTA is an ex­
tremely v e rs a tile reagent, forming s ta b le , soluble stoichiom etric com­
plexes with most metal ions, including a ll o f those lis te d above.
This
very v e r s a t il it y , which makes EDTA such a valuable t i t r a n t , becomes a
problem when s e le c tiv ity is desired;
in a ll pairs in question the metals
w ill in te r fe r e with one another and conditions and reagents must be very
c a re fu lly selected in order to obtain r e lia b le data on in divid ual metal
content.
By judicious u t iliz a t io n o f masking agents, back titr a tio n s
and redox reactio n s, various techniques were patched together making i t
possible to t i t r a t e both metals in one sample fo r any o f the pairs which
did not contain F e ( I I ) .
Before describing the procedures used in the
mixed metal analyses, the simpler methods o f t it r a t in g a sing le metal ion
as in the pure s a lts w ill be discussed.
Cu may be determined by forming the ammine complex and t it r a t in g
w ith EDTA using murexide as the in d ic a to r (1 3 -1 6 ).
The EDTA was
17
standardized against both Cu and Zn solutions prepared by dissolving the
respective metal in acid.
Welcher (13) recommends a saturated solution
o f the murexide, but Elbeih (14) uses a powdered m ixture o f the in d ic a to r
and NaCl.
Previous work in th is lab (15) suggests th a t the la t t e r
method produces a more s a tis fa c to ry endpoint.
Elbeih reports good re ­
s u lts when t it r a t in g 25-100 pg o f m e ta l 9 but found th a t fo r samples
containing less than 25 pg the endpoint became in d is tin c t.
The copper content can also be determined io dom etrically using
Na2S2O3 as the t i t r a n t (1 7 ).
- Standardizations done using e ith e r Cu
ribbon or K2Cr2O7 did not give r e lia b le re su lts a t the micro le v e l;
th e re fo re , KIOg was u ltim a te ly chosen as the primary standard to d e te r­
mine the concentration o f the t i t r a n t .
The Cu containing crystals were
dissolved in doubly d i s t i lle d w ater, buffered with NH^HF2 and t it r a t e d
to the starch endpoint.
To prevent the endpoint from d r if t in g due to
the adsorption o f fre e I 2 on the Cu2I 2 p re c ip ita te , KSCN was added a f t e r
the starch.
T itra tio n s of the pure Cu s a lt without KSCN gave the same
re s u lts , but required more tim e.
In comparative t it r a t io n s both the complexometric and the iodometr ic methods y ield ed the same re s u lts ;
to avoid in terfe ren ce problems
the l a t t e r method was chosen fo r the Cu analyses o f the mixed metal
s a lts , since i t is s p e c ific fo r the C u (H ) ion.
Samples containing only Ni were t it r a t e d with EDTA by the same
procedure described above fo r Cu (1 3 ,1 4 ).
The Co t it r a t io n s performed with EDTA also u t iliz e d the murexide
in d ic a to r prepared in s o lid form as described above.
Welcher (13)
provides a f a i r l y d e ta ile d procedure fo r th is t it r a t io n ;
additional
18
inform ation appears in Houk1s thesis previously c ite d (1 5 ).
The pH
must be c o n tro lled very c a re fu lly in th is t it r a t io n in order to obtain
the correct color change a t the endpoint.
The e asiest way to do th is
was to add a c e tic acid or ammonium hydroxide as needed to maintain a
yellow color p rio r to the endpoint;
a f t e r the endpoint the solution
remains purple regardless o f the amount o f acid or base added.
The
Co t it r a t io n may also be performed as a back t it r a t io n using Eriochrome
Black T (EBT) as the in d ic a to r a t a pH of 10 (1 8 ).
The Mn determination in the pure s a lt was done follow ing the pro­
cedure ou tlin ed by Welcher (13) except th a t the ZnO and KCN were not
added as there were no other metals present to in te r fe r e .
Ascorbic acid
or hydroxyI amine hydrochloride must be added to prevent the oxidation of
M n ( II) .
The system is buffered a t pH = 10;
tr a tio n is again EBT in the s o lid form.
the in d ic a to r in th is t i ­
The M n (II) ion may thus be
t it r a t e d d ir e c tly or an excess o f EDTA may be added and a back t it r a t io n
performed w ith a standard Zn s o lu tio n .
The same basic methods used to t i t r a t e the pure metal compounds .
were u tiliz e d in the mixed s a lt analyses w ith some m odifications to pre­
vent any in terfe ren ce s .
In the Cu containing subseries the Mn-Cu1
Co-Cu and Ni-Cu pairs were f i r s t analyzed fo r Cu io d o m e trica lly .
A fte r
the Cu content was determined, the Co-Cu and Ni-Cu pairs were t it r a t e d
with EDTA fo r to ta l metal content follow ing the procedure described fo r
the pure Cu s a lt .
In the Mn-Cu subseries the Mn was subsequently t t - '
tra te d as in the pure s a lt except th a t KCN was added to form the Cu(CN)^
complex, thus preventing the Cu ion from in te r fe r in g .
In the Mn-Co and Mn-Ni. subseries to ta l metal was found f i r s t , KCN
19
was added to mask the Co and Ni re s p e c tiv e ly , and the lib e ra te d EDTA
was t it r a t e d w ith a standard metal s o lu tio n .
The method described by Welcher (13) fo r determination o f both Co
and Ni simultaneously proved impossible to d u p licate.
A spectrophoto-
m etric method described by Kratochvil and H arris was u t iliz e d to fin d
the mole fra c tio n s o f these two metals in the Co-Ni subseries (2 0 ).
From the t it r a t io n data the r e la tiv e s o lu b ilitie s o f the metals in
th is series can be determined.
For example, in the Cu containing sub­
s e rie s , the Cu ion occupies the metal s ite o f the crystal most re a d ily .
In th is p a rtic u la r subseries the Cu may be regarded as the solvent;
Co and Ni are doped in to the Cu s a lt .
Mn,
Mole fra c tio n data in dicate
th a t the M n (II) ion is the most soluble and the N i ( I I ) is the le a s t s o l­
uble o f the three solute ions.
The Ni analog was the most d i f f i c u l t of
the pure compounds in the e n tire series fo r which to grow crystals and
th is trend is re fle c te d in the low s o lu b ility o f the N i ( I I ) ion in the
mixed metal compounds as w e ll.
In a. solu tion .contain ing equimolar
amounts o f the two metals such th a t xCu so-jn = 0 .5 0 , the c rystals which
form w ill have Xqu x ta -j = 0.74 fo r the Mn-Cu s a lt , 0.96 fo r the Co-Cu
s a lt and 0.99 fo r the Ni-Cu s a lt .
The re la tio n s h ip between Xs0] n and
xXtal fo r the Cu containing subseries is presented in Table IV ;
a
graphic presentation w ill be u t iliz e d in the discussion o f these re ­
s u lts in a .la t e r section.
Table V l i s t s the corresponding mole fra c tio n data fo r the Mn con­
ta in in g subseries.
The Cu is more re a d ily incorporated in to the crys­
ta l stru ctu re than Mn as was observed in the preceding system;
displays a g re a ter s o lu b ility than Ni in th is series as w e ll.
Co
20
Table IV
Mole Fractions in Cu Containing S alts
Cu-Mn Subseries
Cu-Co Subseries
Cu-Ni Subseries
XCu, soln
XCu, x ta l
XCu, x ta l
XCu, x ta l
0.2000
0.1887(25)
0.4057(50)
0.6814(14)
0.2500
0.2825(19)
, 0.6398(12)
0.8489(12)
0.3333
0.5188(44)
0.8330(14)
I
0.5000
0.7500(14)
0.9574(11)
I
0.6667
0.9198(17)
0.9640(24)
I
0.7500
0.9830 (2)
0.9825(24)
I
0.8000
0.9785 (2)
Table V
I
•1
Mole Fractions in Mn Containing S alts
Mn-Cu Subseries
Mn-Co Subseries
Mh-Ni. Subseries
XMn, soln
XMn, x ta l
XMn, xtal
XMn, x ta l
0.2000
0.0215(89)
0.3973(29)
0.7567(14)
0.2500
0.0170(138)
0.5181(10)
0.7745 (2)
. 0.0802(190)
0.6310 (3)
0.8704 ( 6 )
0.3333
.
.
0.5000
0.2500(43)
0,7694 (2) .
0.9440 (3)
0.6667
0.4812(48)
0.8713 (5)
0.9746 ( I )
0.7500
0.7175 ( 8 )
0.8321 (4)
0.9608 (4)
0.8000
0.8112 ( 6 )
0.9386 (5)
0.9757 ( I )
21
. There is also a change in stru ctu re in the Cu containing subseries
which serves to complicate th is system.
The remaining pairs o f a ll the
subseries re ta in the orthorhombic s tru c tu re regardless o f metal compo­
s itio n and in a ll cases Nt displays the le a s t tendency to be incorpor­
ated in to the crystal s it e .
22
MORPHOLOGY AND STRUCTURE
Before discussing the c o lle c tio n o f X-ray data, the shape or habit
th a t these crys ta ls adopt w ill be described.
Taken in conjunction
with the observed dichroism, the habit can provide inform ation as to
the location o f the p rin cip a l axes in a c ry s ta l.
This f a c ilit a t e s the
mounting and o rie n ta tio n o f in divid ual crys ta ls p rio r to taking pre­
cession photographs.
I f no e f f o r t is made to control t h e ir s iz e , stoichiom etric prepa­
rations o f the compounds in th is series y ie ld small a c ic u la r crystals
w ith the needle axis p a ra lle l to the chain d ire c tio n .
T y p ic a lly , these
c rystals are two m illim e ters or less in length and about the thickness
o f a h a ir.
Slow growth w ill sometimes produce a la t h - lik e habit with
dimensions roughly 0 .5 x 1.0 x 2-3 mm.
This la th is an external ex­
pression of the u n it c e ll and is bounded by ( 100) and ( 001) ;
most of
the s tru c tu ra l work w ith the precession camera was done on crystals w ith
th is h a b it.
The needle axis o f the la th lie s along the b^ c ry s ta llo ­
graphic axis and corresponds to the chain d ire c tio n o f the MCl^(H2O) 2
octahedra.
In terms o f chemical bonding and cleavage, th is external
morphology is a lo g ic a l outgrowth of the u n it c e ll o f the c ry s ta l.
The
c c rystallo g rap h ic axis is shorter than the a_ axis and the growth pat­
tern of the laths w ill sometimes r e fle c t th is ;
however, there is no
p a rtic u la r preference fo r a crystal to develop along the £ axis rath er
S
than along a.
The chain axis coincides w ith the needle axis o f a ll
crystals except those in the stru ctu re tra n s itio n region.
Very large crys ta ls 10-15 mm in length and several mm on a side
23
which are grown from s o lu tio n s containin g an excess o f C l" develop
random faces;
h a b its .
even the needle axis may be d i f f i c u l t to locate in such
Figure 3 shows some o f the ty p ic a l habits fo r th is s e rie s .
Figure 3
Some Habits Displayed by the Series TMAMxM^_xC l3*2H20
24
Growth in odd habits was e s p e c ia lly common fo r mixed metal crys­
ta ls which contained Cu.
In these subseries a s tru c tu ra l change from
the orthorhombic Pnma to the monoclinic P2^/c must occur a t some par­
t ic u la r mole fr a c tio n .
In the tra n s itio n region crys ta ls bounded by
( I O l ) 9 ( I O l ) , (H O ) and (H O ) are the ty p ic a l h a b it observed;
shown in Figure 3c.
th is is
In such cases i t was extremely d i f f i c u l t to lo ­
cate the p rin cip a l axes and to o rie n t the crystal fo r X-ray w ork.solely
by morphology.
A fte r mounting, the p rin cip a l d ire c tio n in such
crystals was found more or less by t r i a l and e rro r.
Normally, the crystal was mounted with the needle axis coincident
with the axis o f the goniometer head and a p rin cip al d ire c tio n located
on the basis o f a re fle c te d face.
Crystals with the orthorhombic
stru ctu re were found to display two colors depending on o rie n ta tio n ;
the a-b and b-c planes could be found by. color i f the h a b it o f the crys­
ta l did not happen to contain ( 100) or ( 001) planes which could be lo ­
cated e a s ily .
This made i t possible to o rie n t crys ta ls o f any habit
in the orthorhombic stru ctu re with a minimum o f e f f o r t .
In the Mn-Cu, Co-Cu and Ni-Cu s a lts the habit in Figure 3c predom­
inates when Xqu x ta ] approaches 0 .7 5 -0 .8 0 .
The longest dimension in
these c rystals is along the a_ axis which corresponds to the chain d i­
rectio n in the monoclinic c e ll.
orthorhombic s tru c tu re ;
However, these c rystals re ta in the
a f u l l set o f precession photographs was re ­
quired to demonstrate th a t these mixtures were indeed c r y s ta lliz in g in
the Pnma space group.
At xCu x ta i £ 0.80 the Cu containing s a lts w ill
c r y s ta lliz e in the monoclinic s tru c tu re .
The X-ray data were co llected on a Supper precession camera and
25
Mo Ka ra d ia tio n with X = 0.70926 A was u t iliz e d .
The zero, f i r s t and
second le v e ls o f the pure Mn and pure Cu s a lts were taken shooting in to
the a-b and b-c planes o f lath-shaped c ry s ta ls .
These c rys ta ls were
subsequently remounted and the zero, f i r s t and second le v e ls of the a-c
plane were obtained as w e ll.
This provided a complete set o f preces­
sion photographs fo r both the orthorhombic Pnma and monoclinic P 2 j/c
s tru c tu re s .
As a f i r s t approximation o f s tru c tu re , the zero levels of
any crystal o f the mixed metal s a lts in th is series could now be com­
pared to those o f the pure end members to determine whether or not the
spot pattern matched one or the o th e r.
Full data decks were collected
on some, but not a l l , o f the mixed s a lts as a check to make certain
th a t no anomalies occurred in the upper le v e ls .
The zero and f i r s t
le vels fo r two planes were co llected on a t le a s t three mole fractio n s
o f each subseries.
S alts in the series TMAMClg^HgO where M = Mn, Co and Cu have been
reported in the lit e r a t u r e previously (8 -10 ) and t h e ir structures are
well documented.
Synthesis o f the Fe and Ni end members completes the
5
progression across the f i r s t row tra n s itio n metals from configuration d
to d^ in c lu s iv e .
Spot patterns on the precession photographs in dicate
th a t these two new compounds fo llo w the trend set by the Mn and Co and
adopt the orthorhombic stru ctu re with space group Pnma.
U t iliz in g the
formulas provided by Buerger (21) and Stout and Jensen (22) c e ll dimen­
sions were calculated fo r a ll of the pure compounds from precession data.
These values are s lig h tly lower than those reported in the lit e r a t u r e
fo r the three compounds on which data is a v a ila b le , but these v a riatio n s
are ty p ic a l o f in te rla b comparisons.
For the data co llected in th is
26
work, the Fe and Ni analogs f i t the trend developed w ith decreasing
s ize o f the d iv a le n t metal cations q u ite n ic e ly .
This c o rre la tio n be­
tween the io n ic r a d ii and c e ll dimensions is illu s tr a te d in Table V I.
Table VI
Cell Dimensions fo r Members of the Series TMAMClg'BHgO
Metal
Radius
a
b
£
Mn
0.82
16.59
7.419
8.141
Fe
0.78
16.56 ■
7.331
8.095
Co
0.74
16.52
7.234
8.046
Ni
0.70
16.52
, 7.110
8.031
Cu*
0.73
7.517
7.849
16.70
* axes change in Cu s a lt due to d iffe r e n t space group.
One can also c a lc u la te a th e o re tic a l value fo r b^ based on a square
planar arrangement o f the chloride anions around the central metal atom
and a knowledge o f the ra d ii involved fo r the various ions.
Such a
O
c a lc u la tio n using a value o f 1.81 A fo r the Cl
radius y ie ld s c e ll d i­
mensions which are in remarkably good agreement w ith those observed.
C
Table V II contains these values fo r the d
s e rie s .
Q
through d
analogs o f th is
27
Table V II
Theoretical Values o f j) in Square Planar Arrangement
fo r TMAMCl3- 2H20 in Pnma
Metal
Radius
]d calc
found
Mn
0.82
7.467
7.419
Fe
0.78
7.326
7.331
Co
0.74
7.227
7.234
Ni
0.70
7.071
7.110
28
SPECTRA
The fin a l phase o f the in ve s tig atio n o f the mixed metal series
TMAM^Mj ^Clg'ZHgO e n ta ile d running x.-»
and z-p o la rize d spectra as
w ell as unpolarized spectra o f these crys ta ls on the Cary 14.
When
discussing these spectra, z -p o la rize d lig h t re fers to lig h t which has
it s only allowed v ib ra tio n d ire c tio n p a ra lle l to the z a x is ;
x.,
and
z correspond to the a_, b and jc c rystallo g rap h ic axes in the u n it c e ll.
These o rie n ta tio n s are depicted in Figure 4.
z(c )
Figure .4
Location o f C rystallographic Axes and P o la riza tio n D irections
Crystal spectra were taken a t room temperature in the near in fra re d
( IR ) , v is ib le and u lt r a v io le t (UV) regions.
In the pure Mn compound
three peaks were observed in the UV region, but in a ll other cases th is
region y ie ld ed very l i t t l e
useful inform ation as charge tra n s fe r bands
appeared which were o f f scale.
Most o f the in te re s tin g peaks, were ob­
served in the v is ib le and near IR regions.
In order to run spectra on these c ry s ta ls , specimens a t le a s t a
29
m illim e te r wide had to be grown, but they could not be so th ick as to
e ffe c tiv e ly block a ll transm itted lig h t .
With th is p a rtic u la r series
such dimensions proved d i f f i c u l t to a tta in ;
a d d itio n a lly , the dark
colors and intense absorption bands inherent in some o f the crystals
reduced transmission.
Of the pure s a lts the Mn analog w ith very weak
bands is the most s u ita b le sample on which to take spectra.
The pure
Co and Cu end members must be cut and ground to get th in enough crystals
to produce reasonable spectra.
Pure Fe and Ni c rystals large enough
to record spectra were never obtained.
Among the mixed metal com­
pounds, the e n tire Mn subseries produced crys ta ls which allowed useful
spectra to be recorded with a minimum o f e f f o r t .
This translucence was
due p a rtly to the h a b it, but was probably due in la rg e r p a rt to the d i­
lu tio n e ffe c t imparted to these s a lts by the presence o f the M n (II) ion.
Special sample c e lls had to be improvised to hold the crystal in
place w hile running so lid spectra on the Cary 14.
Brass plates were
cut to f i t in to the normal c e ll holder o f the instrument and holes a
m illim e te r in diameter were then d r il le d , positioned in the center of
the transm itted beam.
The c rystals were taped over the hole o f the
sample p la te , oriented w ith one o f the c rys tallo g rap h ic axes p a ra lle l to
the long dimension o f the p la te .
To obtaiin polarized spectra, s trip s
o f Polaroid film were taped over the holes o f both reference and
sample plates and the o rie n ta tio n and color o f the crys ta l were care­
f u l ly noted.
Many blanks were run w ith ju s t the p la te s , the plates
w ith polarized film in place and w ith d iffe r e n t pieces o f the film to
determine the so rt o f background to expect.
The film has weak bands in
the v is ib le and near IR , but they are very small compared to the crystal
30
sc atte rin g and cannot be mistaken fo r c rys ta l absorption bands in
these regions.
The film does absorb in the UV region;
t h is , along
w ith the presence o f charge tra n s fe r bands, made i t impossible to get
polarized UV spectra.
i
Log In te n s ity
Wavelength (nm)
Figure 5
Polarized Crystal Spectra of TMAMnCl3 -EH2O
Log In te n s ity
32
Wavelength (nm)
Figure 6
Crystal Spectrum o f TMACoClyZHgO in x- or y -P o la rize d Light
Log In te n s ity
33
Wavelength (nm)
Figure 7
Crystal Spectrum of TMACoC I3*ZH2O in Zr Polarized Light
Log In te n s ity
Wavelength (nm)
Figure 8
Polarized Crystal Spectra of TMAMnxCol xCl3- 28^0 (V isible Region)
Log In te n s ity
35
Wavelength (nm)
Figure 9
Polarized Crystal Spectra of TMAMnxC o^xClg• ZH2O
or TMACoClg 'ZHgO (IR Region)
Log In te n s ity
36
Wavelength (nm)
Figure 10
Polarized Crystal Spectra of TMAMnxF e ^ xCl3 eEH2O (IR)
Log In te n s ity
Wavelength (nm)
Figure 11
Polarized Crystal Spectra of TMAMnxN i ^ xCI3- ZH2O (V isible Region)
Log In te n s ity
38
Wavelength (nm)
Figure 12
Polarized Crystal Spectra of TMAMnxN i ^ xCl3 -ZH2O (IR)
Log In te n s ity
39
— ,--------------------------------------1--------------------------------------h
400
500
600
Wavelength (nm)
Figure 13
Polarized Crystal Spectra of TMAMnxCu1^ C l 3* 2H20 (V is ib le Region)
Log In te n s ity
40
Wavelength (nm)
Figure 14
Polarized Crystal Spectra of TMAMnxCu^xCl3 -ZH2O (IR Region)
Wavelength (ran)
Figure 15
Unpolarized Crystal Spectrum
TMAMnxF e ^ xCl3- ZH2O
Log In te n s ity
42
Wavelength (nm)
Figure 16
Unpolarized Crystal Spectrum
TMACoClEHgO
43
DISCUSSION
As mentioned e a r l ie r , th$ o rig in a l in te n t o f th is research was
the synthesis of mixed metal crystals o f the tw o -s ite system TMA3M2C l 7
where M and M1 were expected to assume th e ir preferred geometries.
Prelim inary in ve s tig atio n o f the c rystals grown in th is attempt in d ica­
ted th a ti although they were not the desired stoichiom etry, they were
an in te re s tin g system in t h e ir own r ig h t.
Crystals o f the mixed metal
series a c tu a lly obtained, TMAMxM ^ xC l 3* 2H20 , whdn observed in the p e tri
dishes in which they were grown, displayed two contrasting colo rs,
making i t appear th a t two d iffe r e n t products had formed.
Closer exam­
in atio n o f in divid ual c rystals revealed th a t both colors were v is ib le
in each;
subsequently, t it r a t io n data indicated th a t a mixed metal
product o f f a i r l y uniform composition had indeed been prepared.
The
stru ctu re and composition of these mixed metal compounds are now dis­
cussed in d e t a il.
Dichroism, the v i s i b i l i t y o f two d iffe r e n t colors
w ith in a single c r y s ta l, is discussed in the follow ing section.
I n i t i a l analyses o f the c rystals prepared from solutions containing
equimolar amounts o f the two m etals, plus a stoichiom etric amount of
TMA, suggested a 3:1 preference in moles fo r one metal over the other.
The molar r a tio of chloride to metal was monitored and X-ray pictures
were taken to v e rify th a t the mixtures were a c tu a lly the stoichiom etry
and stru ctu re expected and not double s a lts or defect stru ctu res.
Although there is only one metal s ite a v a ila b le in the hydrated
s e rie s , the existence o f a reproducible 3:1 metal r a tio in the crystals
grown from aqueous solutions with a 1:1 metal r a tio raised the question
as to whether these mixed s a lts could in fa c t be s to ic h io m e tric.
As
44
the range o f mole fra c tio n s in the s ta rtin g solutions was expanded, and
the re s u ltin g products analyzed, i t became evident th a t th is system was
a c tu a lly a s o lid solution with c e rta in re s tric tio n s on the mole fra c tio n
o f a p a rtic u la r metal which could occupy the metal s ite o f the c r y s ta l.
In the experimental section on analyses in Tables IV and V the % values
found in the c rys ta ls were compared to those in the s ta rtin g solutions
fo r various metal p a irs .
Figures 17-19 are graphic representations o f
the same data.
These mixed metal compounds are described in terms o f s o lid solu­
tio n s .
One o f the pure end members may be designated as the solvent .
and the remaining tra n s itio n metal ions are solute p a r tic le s .
I f a ll
the metal cations were id e n tic a l in a ll o f th e ir p ro p e rtie s , and th e ir
fre e energies were the same whether in solution or in the crystal s it e ,
such a system could be used to represent an ideal s o lu b ility curve.
In th is s itu a tio n the x p lo t would be a s tra ig h t lin e w ith slope = 1 ;
the crystals grown would have the same compositiion as the s ta rtin g solu­
tio n .
The observed deviation from lin e a r it y re fle c ts the fa c t th a t the
ions are not a c tu a lly id e n tic a l;
p ro p erties.
they do possess s lig h tly d iffe r e n t
The nature and extent o f the deviation fo r d iffe r e n t
metal pairs allows one to compare the r e la tiv e s o lu b ilitie s o f the
metal ions in th is s e ries .
.
I f the Cu s a lt is chosen as the s o lven t, the mole fra c tio n o f the
solute present in the crystal is represented by:
_
v
% M1, x ta l
nM1 + nCu
'
45
x ta l
Mn Subseries
x Ml , soln
Figure 17
Mole Fraction Plots of TMAMnM'Cl^*ZHgO
46
x ta l
Co Subseries
Figure 18
Mole Fraction Plots of TMACoM'Cl^'ZHgO
47
x ta l
Cu Subseries
xM ', soln
Figure 19
Mole Fraction Plots of TmACuM1CI3-
48
where n re fers to the number o f moles o f the designated metal ion.
Figure 1|9 indicates the re la tio n s h ip between Xjvp Vn the crys ta l and
th a t in the solution fo r the Cu containing subseries.
a ll three curves l i e below the ideal lin e ;
p r e fe re n tia lly in the s o lid phase.
I t is noted that
in each case Cu is taken up
I f one examines the Mn-Cu curve,
i t is seen to possess one p o in t, th a t fo r the highest Mn content, which
a c tu a lly does l i e on the ideal lin e .
Because a ll o f the pure end mem­
bers, where Xm■ - I , do e x is t, a ll o f these curves must approach the
ideal case as xM« ^ 1 -0 .
Thus, even in the real s o lid s o lu tio n s, the
two end points are fix e d ;
what happens to these curves between the end
points is useful in describing the chemistry o f th is system.
The o rig in a l aqueous solutions in the Mn-Cu subseries with xMn=0.20
0.25 and 0.33 produce crys ta ls which e ffe c tiv e ly exclude the solute
m etal, having mole frac tio n s xMn xi;a-] = 0 .0 2 , 0.02 and 0.07 resp ectively
S tructural determinations u t iliz in g data collected with the precession
camera in d icate th a t these mixed metal crys ta ls which are nearly a ll Cu
adopt the monoclinic stru ctu re o f the pure Cu analog.
The ions whose .
pure s a lts grow in the orthorhombic stru ctu re show very l i t t l e
to dope in to the monoclinic system o f lower symmetry;
tendency
only trace a-
mounts o f Mn, Co or Ni are found in crys ta ls of the P2^/c space group.
When Xjvji in the crystal reaches a value o f about 0 .2 0 , the crystal
undergoes a s tru c tu ra l tra n s itio n from P2^/c to Pnma;
c h a ra c te ris tic o f th is region is also e x h ib ite d .
a d iffe r e n t habit
The habit o f the o r­
thorhombic Mn-Cu crystal with the highest Cu content is a d isto rte d
octahedron elongated along a_, the chain d ire c tio n in the monoclinic
c e l l , but the d ire c tio n o f van der Waals forces in the orthorhombic c e ll
49
This habit was shown previously in Figure 3.
In addition to the change
in h a b it in th is tra n s itio n region, twinning commonly occurs in the
Co-Cu and Ni-Cu crys ta ls as w e ll.
I t is in te re s tin g to note how l i t t l e o f the solute metal is neces­
sary to allow the Pnma stru ctu re to predominate.
Only 20% o f the metal
s ite s in the crystal need be occupied by the solute metal ions to enable
the orthorhombic stru c tu re to m anifest i t s e l f .
When th is structure
p re v a ils , the solute metals whose pure s a lts are orthorhombic enter the
crys ta l s ite much more re a d ily than they did in the monoclinic case.
T h e ir s o lu b ility increases sharply follow ing the s tru c tu ra l tra n s itio n
and. ex h ib its the order:
Cu > Mn > Co > N i.
The most s trik in g pattern in the observed order is the decrease in
the s ize o f the metal cation as the s o lu b ility decreases.
Although the
q
C u (II) ion is not the la rg e s t c a tio n , i t has a d electron configuration
and displays a Ja h n -T e lle r e ffe c t when i t occupies the metal s ite o f the
c r y s ta l;
i t thus tends to d is to rt the regular octahedral s ite and ap­
pears to have a la rg e r radius than the numbers would in d ic a te .
This
d is to rtio n is s u ffic ie n t to destroy the Pnma symmetry when more than 80%
o f the s ite s are involved.
Because o f the energy considerations w ith in
the c ry s ta l, the M n (II) ion which is the la rg e s t o f the s e rie s , seems
to pack b e tte r in th is environment than the other sm all6r cations, ex­
cluding Cu.
C o (II) with a sm aller radius would not be expected to sub­
s t it u t e fo r the Cu ion in the la t t ic e s ite s quite as re a d ily as the Mn;
th is is indeed what is observed in the Cu subseries.
considered, N i( I I )
tic e s it e ;
Of a l l the ions
is the most d i f f i c u l t to force in to the crystal l a t ­
i t is also the sm allest.
The pure Ni end member o f th is
*
50
Series tends to form an amorphous powder ra th e r than a c ry s ta llin e soK
id,. and th is e ffe c t is observed in the mixed metal s a lts as w e ll.
Ni
is by fa r the le a s t soluble in the s o lid phase o f the tra n s itio n metals
studied in th is s e ries .
This e ffe c t may be explained by comparing the s ize o f the metal ion
to the hole formed by four chloride anions ju s t touching in a square
planar arrangement as shown in Figure 20.
Figure 20
Packing Hole Formed by Four Chlorides
In order fo r the metal cation to touch these ch lo rid es, it s ionic
radius must be 0.75 A or la rg e r; C o(IT) ju s t f i t s .
N i ( I I ) with an
O
io nic radius o f only 0.70 A is too sm all.
Since th is cation does not
f i t in to the crystal s it e , growth o f the Ni crystal is very d i f f i c u l t to
achieve.
The la rg e r cations such as Fe and Mn, and Cu as w e ll, tend to
push the chloride anions apart when they occupy the S it e 9 thus reducing
the e le c tro s ta tic repulsions among the Cl" ions.
Absolute values fo r the ra d ii o f the ions involved in th is exer­
cise are impossible to determine.
How well the actual numbers in these
51
calculatio ns f i t the observed s o lu b ilitie s is therefore dependent on the
choice o f r a d ii one makes.
Those calculated by Shannon and P rew itt
(11) take in to account the spin and coordination number o f the metals
and were therefo re preferred over Pauling's values.
As i t turned ou t,
these values also in d ic a te qu ite w ell th a t N i ( I I ) is too small to occupy
the crystal s ite in th is s e rie s .
the trend remains the same.
Regardless o f whose r a d ii are chosen,
N i ( I I ) is always the sm allest and predic­
ta b ly the most d i f f i c u l t to force in to the crystal s it e .
Based on the s o lu b ilitie s observed in the Cu containing subseries
and the ra tio n a le proposed to account fo r these observations, one would
p red ict th a t in the Mn subseries Cu w ill again be more soluble than Mn,
and th a t Co w ill dissolve more re a d ily than Ni with the l a t t e r two both
less soluble than Mn.
s e rie s .
This is borne out in the x plots of these sub­
In the Co subseries both Cu and Mn are more soluble than Co
which is indicated by the p o sitiv e deviations from the id e a l.lin e fo r
these curves;
Ni has a negative deviatio n showing th a t i t once again
is the le a s t soluble o f the tra n s itio n metal ions in th is s e ries .
A spectrophotometric technique (20) was u tiliz e d to determine both
Co and Ni simultaneously in a single sample;
th is a n a ly tic a l method
was not s e n s itiv e enough to detect very low concentrations o f N i,
y ie ld in g xNi x ta l values o f zero fo r xN i^ oln values less than 0 .50.
This method does in d ic a te a t what mole fra c tio n in the aqueous solution
appreciable amounts o f Ni begin to appear in the re s u ltin g c ry s ta ls .
The technique, although not as s e n sitive as the t it r a t io n methods, is
s u ffic ie n t to corroborate the e x is tin g s o lu b ility p a tte rn .
F a m ilia rity with th is system allows one to a c tu a lly rank a group of
52
crystals from the same subseries according to r e la tiv e metal content by
visual examination alone.
Although d i f f i c u l t to describe q u a n tita tiv e ­
ly , a gradual, subtle change in color is observed in a group o f crystals
in the orthorhombic stru ctu re as mole fra c tio n changes.
One must ex­
ercise care in selecting crys ta ls o f roughly the same s ize and q u a lity
when doing such a comparison, however.
C rystals from solutions which
contain progressively more Ni e x h ib it a gradual, but d e f in it e , color
change i f they are compared to one another, supplying fu rth e r evidence
th a t more Ni is indeed being dissolved in the s o lid s o lu tio n .
These
q u a lita tiv e observations on the crys ta ls can be q u an tifie d by a nonde-s tru c tiv e method, u t iliz in g the precession photographs.
As one d is ­
solves more o f the solute metal ion in to a c r y s ta l, not only the color
but also the c e ll dimensions should change gradually as a. re s u lt of the
d iffe r e n t sizes o f the two ions.
X-ray data in d ic a te th a t th is e ffe c t can indeed be seen.
For a
p a rtic u la r subseries o f the mixed metal c ry s ta ls , the values of a_» j) and
£ should vary in a p red ictab le way with the molar, r a tio o f the component
metals.
A large d iffe re n c e in ra d ii occurs between the M n (II) and
C u (II) ions;
thus, th is e ffe c t should be noticeable in th is subseries.
However, a s tru c tu ra l tra n s itio n also occurs in th is case which lowers
the range o f Xqu which can be studied.
Cell dimensions fo r orthorhom­
bic c rystals o f the Mn-Cu subseries w ith xCu = 0.20 and Xqu = 0.75 are
presented in Table V I I I on the follow ing page.
The Mn-Co subseries also displays a f a i r l y wide range o f mole fr a c ­
tio n s , Xq0 = 0.05 to Xq0 = 0 .6 0 , and there is no change in structure
between end members to complicate m atters.
Table IX compares the c e ll
53
dimensions o f the extreme x values o f the Mn-Co subseries to those of
the pure end members.
Table V I I I
Cell Dimensions o f Some C rystals in the Mn-Cu Subseries
XCu
0.00
Metal
. Mn
a_
b
C
16.78
7.434
8.227
0.20
Mn^Cu
16.64
7.469
8.095
0.75
Mn-Cu
16.59
7.419
7.828
1.00
Cu*
16.73
7.479
■
7.864
* Cu axes redefined, a-*b, b->c, c-*a
Table IX
Dimensions o f Some Crystals in the Mn'-Co Subseries
Metal
a
0.00
Mn
16.78
7.434
8.227
0.05
Mn-Co
16.76
7.426
8.213
0.60
Mn-Co
, 16.61
7.277
8.085
1.00
Co
16.52
7.234
8.046
xCo
b
C
The Mn-Co c rystals seem to f i t the expected trend in c e ll dimen­
sions.
The crystal which is nearly pure Mn has values which are very
close to those fo r pure Mn, and the crystal with Xq0 = 0.60 has values
54
which are approaching those fo r the pure Co end member.
In the case o f
the Mn-Cu c ry s ta ls , th a t with a low mole fra c tio n o f Cu f i t s the trend,
but the crystal which is predominantly Cu is anomalous.
This mole
fra c tio n also occurs in the region in which the habit is q u ite d iffe r e n t.
The preference fo r th is new habit may be re la te d to some in te ra c tio n a t
local s ite s between the Mn and th e .Cu cations which could also give ris e
to the break in the pattern o f c e ll dimensions which is noted in th is
p a rtic u la r region.
An anomaly may well be.expected fo r these crystals
in -this region o f s tru c tu ra l and habit tra n s itio n s .
For many o f the
observed patterns in th is work the C u (II) ion has proven anomalous.
The dichroism exhibited by these crys ta ls has been mentioned ear­
l i e r , and i t is th is property above a ll else which f i r s t captures the
a tte n tio n and in te re s t o f the observer.
One o f the special a ttra c tio n s
o f the mixed metal series is the s trik in g v i s i b i l i t y of two and some­
times three d iffe r e n t colors w ith in a sin g le c ry s ta l.
This dichroism
may be characterized by visual examination o f the c ry s ta ls , by observa­
tio n o f the c rystals w ith a p o la rizin g microscope, or by comparison of
polarized crystal spectra in various o rie n ta tio n s .
The wavelength at
which an absorption maximum occurs has an e ffe c t on the value o f the
r e fra c tiv e index, n, determined fo r a c ry s ta l.
The index o f re fra c tio n of a substance, defined as the r a tio of the
v e lo c ity o f Tight in a vacuum to th a t in the substance, is one o f the
most basic physical properties measured by chemists.
The substance of
in te re s t is usually a liq u id , but may also be a gas or a s o lid .
Meas­
urement o f the r e fra c tiv e index o f a c r y s ta llin e s o lid is o f p a rtic u la r
in te re s t to the m ineralogist (2 3 -2 5 );
the physical method used to
55
determine n is necessarily d iffe r e n t fo r a s o lid than a f lu id .
In
addition to the problems associated with a more involved procedure,
d iffe r e n t values may be obtained fo r n depending on the o rie n ta tio n i f
the s o lid ex h ib its anisotropy.
The v e lo c ity o f lig h t m ay.actually vary
depending on what d ire c tio n i t is tra v e lin g through the c ry s ta l.
Crystals are c la s s ifie d as is o tro p ic or aniso tro p ic.
In an iso ­
tro p ic crystal the v e lo c ity o f li g h t , or the r e fra c tiv e index, is the
same in a ll d ire c tio n s , as is the case fo r liq u id s and glasses.
If a
three-dimensional surface in d ic a tin g the value o f the re fr a c tiv e index,
n, in any d ire c tio n w ith in the crystal were constructed fo r an is o tro p ic
c r y s ta l, i t would be s p h e ric a l;
a trix .
such a surface is defined as an in d ic -
Perhaps the most common example o f an is o tro p ic crystal is
ta b le s a lt , NaCl.
This is a cubic system, the highest possible symmet­
ry class, and is completely described i f one c e ll dimension is known;
to use a chemist's expression, i t has one degree of freedom.
On the
basis of the s tru c tu ra l symmetry o f NaCl, i t should not be surprising
th a t a cubic crystal has only one value o f n.
In anisotropic c rystals the v e lo c ity o f lig h t varies depending on
the o rie n ta tio n o f the c ry s ta l.
Such crys ta ls can be described in
terms of t h e ir in d ic a tric e s as e ith e r u n iaxial or b ia x ia l.
In uniaxial
c rystals there is one, and only one, axis defined as the o p tic axis
which is perpendicular to a c irc u la r or is o tro p ic cross section o f the
in d ic a tr ix ;
in b ia x ia l c rystals there are two such axes.
The uniaxial
crystal has the higher symmetry and belongs to e ith e r the te tra g o n a l,
hexagonal or rhombohedral class.
Two degrees o f freedom must be spec­
if ie d to completely define these systems;
two edges, a_ and £ , fo r the
A
56
f i r s t two and one edge and one angle fo r the la s t one.
In a tetragonal system j* = b;
thus, symmetry requires a 4 -fo ld
p rin cip al axis p a ra lle l to the £ axis o f the u n it c e ll which w ill coin­
cide with the o p tic axis o f the in d ic a t r ix ..
longer than or shorter than the a^ and Jd axes.
The £ axis may be e ith e r
The e llip s o id which
represents the in d ic a trix may also be p ro late (n£ > n^) or oblate
(n£ > nQ) ;
c ry s ta l.
or
m ineralogists use th is d is tin c tio n to define the sign o f a
A tetragonal crystal may be e ith e r uniaxial p o s itiv e , U (+ ),
negative, U( - .) , as shown in Figure 21.
Figure 21
Tetragonal In d ic a tric e s
’
The sign may a c tu a lly be determined with a microscope w ithout knowledge
o f the absolute values o f n and n .
e
o
While the c e ll dimensions define
the o p tic a x is , they y ie ld no information on the r e la tiv e values of n.
The m in e ra lo g is ts 's most dramatic example o f a u n iaxial crystal is
Iceland spar, more commonly c a lle d cal c it e .
C alc ite is calcium carbon­
a te , CaCOg, which f a lls in the rhombohedral class.
crystal which cleaves n a tu ra lly in to rhombs.
I t is .a transparent
I f one o f these rhombs
57
is placed over a mark on a piece o f paper, two marks are v is ib le through
the c ry s ta l.
This is shown in Figure 22.
Figure 22
Double Refraction o f Iceland Spar
One of the observed images is produced by the ordinary ray, the other by
the extraordinary ray.
They can be distinguished by ro ta tin g the
rhomb as i t lie s on the paper;
the ordinary image remains fix e d while
the extraordinary image rotates with the c r y s ta l.
A close examination
o f the two images reveals th a t both appear higher than the paper which
is to be expected due to the d iffe re n c e in r e fra c tiv e index between a ir
and the c ry s ta l.
appears a l i t t l e
However, the image produced by the extraordinary ray
lower than th a t from the ordinary ray.
The d iffe r e n t
apparent depth is evidence fo r two r e fra c tiv e indices w ith in a single
crystal in th is o rie n ta tio n .
This illu s t r a te s the property o f double
re fra c tio n exhibited by a ll anisotropic c ry s ta ls .
M ineralogists quan­
t i f y th is property by defining a term, b ire frin g e n c e , which is the d i f ­
ference between the maximum and minimum r e fra c tiv e indices.
58
The la te r a l lo cation and ro ta to ry motion o f the extraordinary
image depends on the wave nature o f lig h t and the a b il it y o f a crystal
to resolve lig h t in to two m utually perpendicular v ib ra tio n d ire c tio n s .
Light is electromagnetic ra d ia tio n ;
to r,
ordinary lig h t has an e le c tr ic vec­
capable o f v ib ra tio n in any d ire c tio n perpendicular to the prop­
agation d ire c tio n .
In the cal c ite crystal a ll o f these v ib ra tio n d i­
rections are resolved in to two orthogonal d ire c tio n s .
The vib ratio n
d ire c tio n which represents t normal to the o p tic axis is th a t o f the
ordinary ray.
The E vector o f the ordinary ray is in the plane o f the
c irc u la r cross section o f the in d ic a tr ix ;
th is means th a t the e le c tric
f ie ld o f th is ray in te ra c ts with th a t o f the electrons in the crystal
in the same fashion in any d ire c tio n w ith in the a-b plane.
The ordin­
ary ray propagates through the crystal as i f i t were i is o tro p ic .
The E vector o f the extraordinary ray is perpendicular to th a t o f
the ordinary ray and in the same plane as the optic a x is .
By symmetry
the atomic arrangement along c is d iffe r e n t than th a t along a or Jd.
The vibration s due to the electrons in th is asymmetrical environment
in te rfe re with t o f the extraordinary ray and produce a re s u lta n t vector
which is no longer exactly perpendicular to the fay d ire c tio n . . However,
the propagation d ire c tio n must s t i l l be perpendicular to the E vector;
th is so-called wave normal no longer coincides with the ray as i t does
in the ordinary ray.
dinary ray s lig h t ly ;
rhomb.
This in te ra c tio n bends the path o f the extraor­
thus, two images appear as lig h t e x its the cal c ite
The paths of these two rays are shown in Figure 23.
59
ordinary ray
. extraordinary ray
Figure 23
Ray D irections and Wave Normals
The re s o lu tio n , i . e . the distance between these two images, is d i­
re c tly proportional to the birefringence and to the thickness of the
c ry s ta l.
In cal c ite lig h t tra v e ls fa s te r along the op tic a x is ;
th is case, n^ < nQ.
in
This means th a t n£ is closer to nai-r than nQ so
th a t the extraordinary image should appear lower than the ordinary one
which is indeed the case.
A th ic k e r rhomb w ill cause the two images
to appear fu rth e r apart due to the longer path length the two rays
fo llo w .
This same process occurs in a ll anisotropic c ry s ta ls .
Cal-
c ite is unusual in th a t i t is transparent and has an extremely high
b irefrin g en ce.
Quartz w ill also form two images, but a crystal 15
times as th ic k is required to observe the same separation o f images.
I t is possible to observe anisotropic crystals with lig h t which .
has only one allowed v ib ra tio n d ire c tio n ;
such lig h t is " p o la riz e d ."
Plane polarized lig h t may be obtained in a number of ways;
some
60
crys ta ls such as b io t it e or tourmaline n a tu ra lIy p o la rize lig h t .
A
Nicol prism often used in p o la rizin g microscopes consists o f a cal c ite
crystal which is oriented so th a t only the c ry s ta l's extraordinary ray
is allowed to pass through..
Large sheets of P olaroid, the substance
which was used to obtain the polarized spectra in th is work, are made
by embedding oriented c rystals o f herapathite in a p la s tic sheet.
Even
lig h t re fle c te d from a b rig h t, nonmetal surface is polarized to some
e x ten t.
I f the two images produced by cal c ite are.observed through a po­
la r iz e r as the rhomb is ro ta te d , each one w ill disappear or extinguish
a lte rn a te ly a t 90° in te rv a ls .
The plane o f the v ib ra tio n d ire c tio n of
the ordinary and extraordinary rays can thus be determined.
I f there
is no vector component o f the ordinary ra y , th a t image w ill not be
seen;
i t extinguishes or is a t an e x tin c tio n po sitio n .
There are two
such positions fo r the extraordinary ray as w e ll, 180° apart and orthog­
onal to those fo r the ordinary ray.
A ll the properties th a t have been discussed fo r un iaxial crystals
are also present in b ia x ia l c ry s ta ls , but the la t t e r show other e ffec ts
as well due to t h e ir g reater complexity.
Crystals which f a l l into the
orthorhombic, monoclinic or t r i c l i n i c classes are b ia x ia l.
crystals the in d ic a trix is a t r i a x ia l e llip s o id ;
fe re n t value fo r n.
For such
each axis has a d i f ­
The two extremes are denoted as n^ fo r the lowest
value and n^ fo r the highest value.
The b ia x ia l in d ic a trix has two
op tic axes which are normal to c irc u la r cross sections whose ra d ii rep­
resent n^, the interm ediate re fra c tiv e index fo r b ia x ia l c ry s ta ls .
The location o f these indices w ithin the in d ic a trix is illu s tr a te d in
61
Figure 24.
n
Y
n
a
radius of c ir c le = n
Figure 24
B iaxial In d ic a trix
In orthorhombic c rystals symmetry requires th a t the three axes of
the in d ic a trix coincide w ith the three crystallographic axes, but no re ­
s tric tio n s are placed on the actual mapping, i . e . the shortest c ry s ta l­
lographic axis need not correspond to n .
is fy th is requirement;
found by experiment.
There are s ix ways to s a t­
the correct one fo r a p a rtic u la r system is
In the monoclinic system one o f the 2-fo ld axes
o f the in d ic a trix w ill coincide with the unique axis of the c r y s ta l,
In the t r i c l i n i c system any o rie n ta tio n is possible.
The differences in symmetry along the various axes in anisotropic
crys ta ls are b a s ic a lly a re s u lt of d iffe r e n t atomic arrangements.
This
resu lts in the v a ria b le re fra c tiv e indices observed in such c ry s ta ls .
The in te ra c tio n o f the crystal with lig h t is also what determines the
color o f a c r y s ta l.
An anisotropic crystal is therefo re capable of
displaying, d iffe r e n t colors depending on the o rie n ta tio n and d ire c tio n
in which i t is observed.
I f a ll three axes in a b ia x ia l crystal have
62
a d iffe r e n t associated c o lo r, the general term fo r the phenomenon is
pleochroism.
I f only two colors are involved, i t is ca lle d dichroism.
The la t t e r term is often used fo r a ll cases, since in any one o rie n ta ­
tio n o f the crystal only two colors can be observed.
Tourmaline which grows as elongated prisms is an example o f a d ichroic c ry s ta l;
rized lig h t .
i t has been mentioned previously as a source of pola­
This mineral allows lig h t v ib ra tin g p a ra lle l to the prism
axis to pass through w hile completely blocking lig h t v ib ra tin g normal
to the prism a x is .
It s dichroism may be described as black and w hite.
Two c rystals of tourmaline with th e ir prism axes orthogonal w ill not
allow any lig h t to pass.
Most anisotropic crystals are dichroic or pleochroic to some ex­
te n t, but cases among minerals strong enough to see, p a rtic u la r ly with
contrasting c o lo rs, are very ra re .
is one such example;
Andalu s ite , an aluminum s ilic a t e ,
some crystals of th is mineral which contain a
small amount o f im pu rities are y ello w , green and red along the p r in c i­
pal axes o f it s in d ic a tr ix .
When viewed along any p rin cip a l a x is , the
color observed is the sum o f the colors o f the other two axes.
In colored substances such as the dichroic and pleochroic minerals
discussed above, the re fra c tiv e indices have unusually high or low v a l­
ues due to the presence of absorption bands in the v is ib le region.
Because the r e fra c tiv e index is a function of wavelength, X, values of
n are reported with reference to some known wavelength o f lig h t used to
make the measurement, fo r example the Na D lin e .
The follow ing equa­
tio n can be used to describe the re la tio n s h ip between the re fra c tiv e
index, n, and wavelength, X (26):
63
A1 X2
• n2 = I +
--------------
■
where A1 is a constant and X1 is an absorbing wavelength.
(I)
I f a sub­
stance has more than one absorption band, addition al terms involving X
are added.
quation.
There are three cases which can be considered fo r th is eI f the wavelength used to determine n is much greater than
the absorbing wavelength, X »
X1 , Equation I reduces to:
n2 = I + A1 = k
where k is the d ie le c tr ic constant.
( 2)
This equation generally holds
fo r gases and dispersion is not usually observed.
I f X > X1 , but the r a tio X1ZX is sm all, Cauchy's Theorem fo r d is ­
persion re s u lts :
2
A, * ?
n - I + A, + ----- (3)
1
X^
Let A = I
+A1
B = A1X1
2
B
n =A + — ^
X^
which is Cauchy's Theorem.
(4)
I f n is p lo tted against X, a normal dispersion curve re su lts from Equa­
tio n 3 with n increasing gradually as X decreases.
As the wavelength used to measure the re fra c tiv e index approaches
the absorbing wavelength. Equation I requires th a t n become very la rg e ,
going to i n f in i t y a t X = X1 .
Since th is cannot a c tu a lly occur, the
expression does not adequately define n a t th is p o in t.
A damping term
must be added to the equation in order fo r i t to apply to a ll values of
64
A, including absorption points (2 7 ).
The Mn, Fe, Co and Ni s a lts as well as th e ir mixed metal analogs
prepared in th is research a ll c r y s ta lliz e in the orthorhombic class
with space group Pnma, and are thus examples o f b ia x ia l c ry s ta ls .
Be­
cause they are highly colored, they possess v is ib le absorption bands
which w ill give ris e to anomalous dispersion curves.
is t ic s ,
These character­
lower than cubic symmetry and absorption bands in the v is ib le
region, are two requirements a crystal must meet in order to be dichroic .
The th ird and cru c ia l requirement is th a t the local s ite symmetry
o f the chromophore be oriented or aligned in the u n it cel I so th a t the
dichroic e ffe c t is not cancelled in the same way as o p tic al a c t iv it y in
a racemate.
The MCl^(H2O) 2 octahedra in the Pnma stru ctu re and in the
P2-|/c stru ctu re as well also meet th is th ird requirement;
th e re fo re ,
the c rystals in the TMAM^Mj_^C ly2H 20 series are d ich ro ic .
The stru ctu re and c e ll dimensions o f these compounds have been
discussed e a r lie r .
To review b r ie f ly , the axes and corresponding
bonding d irectio n s are shown in Figure 25.
z (c )
x(a)
Figure 25
U nit Cell Axes and Bonding D irections
65
Of the pure orthorhombic s a lts , the Co analog displays the most
re a d ily v is ib le dichroism without the aid o f a p o la riz in g microscope,
although the c rystals must be selected ju d ic io u s ly fo r the colors to be
observed.
L ite ra tu re reports these c rystals to be blue in ^ -p o la rize d
lig h t propagating along the a^ a x is ;
c rystals appear r e d -v io le t.
th is work.
in a ll other o rien tatio n s these
These observations have been confirmed in
The dichroism o f the Mn and Cu analogs has
not been re ­
ported in the lit e r a t u r e previously.
Crystals in the mixed metal subseries containing Co are a ll s t r i k ­
in g ly d ich ro ic ;
Table X summarizes th e ir behavior when observed in po-^
la riz e d and in unpolarized lig h t .
Light designated as ^ -p o la rize d has
it s allowed v ib ra tio n d ire c tio n , which corresponds to ? , p a ra lle l to
the a_ c rys tallo g rap h ic a x is , s im ila rly fo r the jD- and ^ -p o la rize d lig h t .
I f the Co containing compounds were observed only in unpolarized
lig h t , these crys ta ls would be classic examples of dichroism and pleochroism.
In a crystal o f the Mn-Co s a lt the shorter wavelengths of
v is ib le lig h t polarized in the ^ or Ib d ire c tio n are absorbed, and the
crystal appears r e d -v io le t.
When c_-polarized lig h t is propagated
through the b-c plane o f the c r y s ta l, the longer wavelengths are ab­
sorbed, and the crystal appears blue.
The iso tro p ic behavior in the
a-b plane can be ra tio n a liz e d on the basis o f s tru c tu re , since the metal
ion giving r is e to the absorption band is in an e s s e n tia lly square p la ­
nar environment with the the surrounding Cl" ions.
In the b-c plane,
however, the metal is bound to HgO in one d ire c tio n and to Cl in the
o th e r, qu ite a d iffe r e n t atomic environment and thus a d iffe r e n t color
may be expected fo r c -p o la r i zed lig h t than fo r b -polarized lig h t .
In
66
Table X
Colors Exhibited by TMACoxMJ ^ ly Z H g O Crystals
Crystal
P o la riza tio n
pure Co
a-b Face
b-c Face
none
re d -v io le t
blue
a
re d -v io le t
b
re d -v io le t
blue
C
Co-Mn
none
red
a .
red
b
red
.
none
red
a
lig h t red
b
dark red
none
dark red
a
dark red
b
dark red
c
red
blue
dark red
blue
C
Co-Cu
blue
blue
C
Co-Ni
r e d -v io le t
green
dark red
green
67
such a stru ctu re dichroism seems to be a lo g ic a l r e s u lt.
With the aid
o f an instrument pleochroism can be demonstrated fo r these s a lts which
are a c tu a lly orthorhombic in symmetry.
I f the p o la riz e r is removed, the color o f the b-c face should be
r e d -v io le t, the re s u ltin g combination o f red and blue.
the case.
This is not
The b-c planes o f the pure Co and the Mn-Co mixed s a lt are
both blue when viewed e ith e r in ^ -p o la rize d lig h t or unpolarized lig h t .
This is not normal dichroic behavior.
The Cu-Co s a lt shows the same
anomalous persistence o f the color green in it s b-c plane.
In the
section on the spectra o f these compounds which fo llo w s , i t is noted
th a t the red absorption bands are indeed present in the unpolarized
spectra.
I t seems th a t the red colo r along the Jb axis is not intense
enough to mask or even to mix appreciably with the blue color of the £
a x is .
This problem is discussed in d e ta il below.
The Co-Ni crys ta ls are v is ib ly pleochroic without the aid of an
instrument.
With polarized lig h t propagating perpendicular to the a-b
plane, two shades of red are observed.
This is re a d ily apparent i f
two c rystals w ith th is o rie n ta tio n are placed a t r ig h t angles to one
another as in Figure 26.
The crystal with it s j) axis p a ra lle l to t
is dark red whereas the crystal with it s £ axis p a ra lle l to t is a much
lig h te r red.
In a d d itio n , i f the p o la riz e r is ro ta te d , one red fades
and the other darkens, u n til a t 45° the two shades o f red appear iden­
tic a l.
This same e ffe c t is seen in the b-c face of a c rys ta l with
the colors dark red and blue;
a t 45° both crys ta ls appear purple.
With these c rys ta ls i t is d i f f i c u l t to be c e rta in th at the red is ac­
tu a lly dark red due to o rie n ta tio n , because the c rystals tend to be
68
■' c
a t 0°
same
same <
v io le t
a f t e r ro ta tio n o f 45°
a fte r ro ta tio n of 90°
Figure 26
Colors Observed in Co-Ni Crystals in Polarized Light
69
th ic k e r along c than along _a which w ill also deepen the color observed.
However, symmetry requires th is red to be the dark red observed in the
a-b face.
With three d iffe r e n t colors v is ib le depending on o rie n ta ­
tio n , th is system is a classic case o f pleochroism.
In unpolarized lig h t the blue color again p e rs is ts .
Spectral
data in d ic a te th a t two d iffe r e n t reds ought to be d is tin g u is h a b le , but
unpolarized spectra do not show only blue peaks fo r the b-c plane.
The subseries containing Mn also ex h ib its these phenomena;
colors are tabulated in Table X I.
the
Both the Mn and Mn-Ni analogs are
pink in a-p o larize d lig h t and c le a r in depolarized lig h t .
.
In ^ -p o la ­
rize d lig h t the pure Mn crystal remains c le a r, but the Mn-Ni mixed metal
crystal appears yello w .
the Co-Ni analog.
The l a t t e r is thus v is ib ly pleochroic as is
However, i t is extremely d i f f i c u l t to distinguish
between the yellow and c le a r o rie n ta tio n s unless the crys ta ls are about
fiv e m illim e ters th ic k or there are c rys ta ls the same s ize with which
to compare colors.
The same technique used to separate the two reds
in the Co-Ni case can be applied to th is system.
The Mn-Cu analog is dichroic in the true sense o f the word as the
m ineralogist uses the term.
Two colors are v is ib le in polarized lig h t ;
amber in ar or b^-polarized lig h t and green in ^ -p o la rize d lig h t .
In
unpolarized lig h t the color observed is a mixture o f the two seen in the
two polarized d ire c tio n s .
This may also be true of the Mn and Mn-Ni
s a lts , but i t is impossible to t e l l v is u a lly because one o f the "colors"
involved is transparent.
The o p tical s p e c t r a ,p a r t ic u la r ly in the v is ib le region, o f spe­
cies w ith tra n s itio n metal ions are generally described in terms of d-d
70
Table XI
xM
i-xCV 2H2°
Colors Exhibited by TMAMn
Crystal
P o la riza tio n
pure Mn
none
a-b Face
pink
a
pink
b
c le a r
none
pink
a
pink
b
c le a r
none
pink
a
pink
b
yellow
none
amber
a :
I t amber
b
dk amber
C
c lear
c le a r
c le a r
y e l I ow
yellow
c le a r
C
Mn-Cu
c le a r
c le a r
C
Mn-Ni
b-c Face
c lear
C
Mn-Fe
Crystals
amber
dk amber
green
71
tra n s itio n s which have energies on the order o f IO^ cm""*'.
The in ten ­
s itie s o f peaks due to d-d tra n s itio n s are weak as a consequence o f
th e ir v io la tio n o f the selection ru le , A l = I l ;
in complexes which
contain high spin M n (II) these tra n s itio n s are spin forbidden as w e ll.
In the high energy v is ib le and UV regions these d-d peaks may be hidden
by the more intense chloride to metal charge tra n s fe r bands.
Three d iffe r e n t approaches have been u t iliz e d to explain the ob­
served spectra o f tra n s itio n metal complexes;
these are crystal f ie ld
theory, CFT, ligand f ie ld theory, LFT, and molecular o rb ita l theory,
MOT.
The f i r s t assumes th a t the central metal ion and a ll surrounding
ions are point charges which undergo purely e le c tro s ta tic in te ra c tio n s ;
no overlap o f o rb ita ls is allowed to occur.
LFT is a refinement on
th is and allows some in te ra c tio n or overlapping of o rb ita ls to occur
between the metal and neighboring atoms.
In the la s t approach the
atomic o rb ita ls o f the ligands and the metal in te ra c t to form molecular,
o rb ita ls which may be constructed by group th e o re tic a l methods so as to
a tta in maximum overlap.
Because o f the nature o f the metal ions and ligands involved in
th is work, CFT was selected as the best approach to explain the ob­
served spectra.
Whatever approach is used, the in te rp re ta tio n o f the
spectra requires some knowledge o f the i n i t i a l and fin a l s ta te s .o f the
system.
The to ta l energy o f a normalized wavefunction is expressed
by Equation 5:
E = J V f H ip d?
=
<^|H|^>
The Hamiltonian op erator, H, fo r a many electron wavefunction which
could be used to describe a tra n s itio n metal ion is a sum o f several
(5)
72
terms:
r
Z Vt 2m
z
Z V,
( 6)
.i> j r U
Z V.
(7)
The f i r s t three terms are the k in e tic energy of the electrons', the
coulombic a ttra c tio n between the nucleus and the electrons and the in te re le c tro n ic repulsions, re sp ec tiv e ly .
Because these terms, are iden­
tic a l fo r ions with a 3d subshell, they may be collected and defined as
H°.
CFT defines V^. as the p o ten tial f ie ld introduced by a p a rtic u la r
environment such as octahedral, Vo c t, or te tra g o n a l, Vt e t .
In the
metal complexes considered here, V. is small compared to the e lectro n electron repulsion term, arid may thus be treated as a perturbation of
H°.
To determine the r e la tiv e energy level s p littin g o f the d o rb ita ls
in various complexes only the e ffe c t o f V. need be considered.
The methods of group theory (28-30) provide a powerful tool in the
analysis o f spectra without recourse to the tedious computations which
solving Equation 5 e n ta ils .
tonian;
Symmetry operator
commute with the Hamil­
i f the d ire c t product o f the characters of the terms in any in ­
tegral o f the form <ip|0 |^> where 0 is an operator does not contain the
t o t a lly symmetric representation, the in te g ra l has a value o f zero.
Use of the symmetry o f a complex and the character ta b le fo r it s assoc­
ia te d point group y ie ld s a great deal o f information about the possible
tra n s itio n s which may be observed.
The fiv e d o rb ita ls o f the tra n s itio n metal ions may be chosen as
a basis s e t;
metry.
these are a ll degenerate in the free ion in spherical sym­
I f the metal ion is surrounded by ligands, the fiv e -fo ld
73
degeneracy is removed in a way which depends on the symmetry o f the com­
plex.
When six id e n tic a l ligands are coordinated in octahedral sym­
metry, the point group fo r the complex is
.
In
symmetry the fiv e
degenerate d o rb ita ls are s p li t into two energy levels designated e and
9
2
2 2
tgg which contain the z , x -y and xy, xz, yz d o rb ita ls re sp ec tiv e ly .
I f two axial ligands are removed or replaced, the symmetry of the com­
plex is lowered to D ^ .
In th is environment a ll degeneracy is removed
from the octahedral eg le v e l;
(dx2_y2).
i t s p lits in to an aig ( dz2) and a b ^
The tgg level becomes a b2g( dXy ) and a doubly degenerate
eg(dxz and dy Z)*
I f the symmetry is lowered fu rth e r to the
point
group, a ll degeneracy is removed so th a t each d o rb ita l has it s own as­
sociated energy level and is described by a one-dimensional representa­
tio n from th is group. . These s p littin g patterns are shown in Figure 27.
Spherical
. D,
lig
b
ag
dz 2
ag
d*V
Ig
Zg
b
d
b
3g
d
yz
Zg
Figure 27
d
xy
XZ
S p littin g o f the d O rb ita ls in Various Symmetries
74
A ll of the above s p lit t in g assignments may be obtained d ir e c tly from the
appropriate point group or can be derived by performing the operations
o f the point group on the in divid ual d o rb ita ls and comparing the re s u l­
tin g representations with those o f the group.
Group theory indicates the possible energy le v e ls , but does not
enable one to estab lish the extent o f the s p littin g and thus the r e la ­
tiv e energies o f the re s u ltin g le v e ls .
This s p lit t in g may be deter­
mined w ithout a c tu a lly solving Equation 5 in part by the location o f the
ligands w ith respect to the lobes o f the d o r b it a ls , but is also depen-
dent on the nature o f the ligands involved.
Ligands are described as
weak or strong f ie ld based on, the magnitude o f the s p lit t in g which they
induce between energy le v e ls ;
the octahedral s p littin g between eg and
tgg is defined as A or IODq.
Complexes containing weak f ie ld ligands
generally possess high spin, because less energy is required to occupy
the higher o rb ita l than to p a ir spins in the lower one.
In the c rystals involved in th is work the metal ions are octahe­
dral Iy coordinated, M C l^ ^ O ^ , but with water molecules occupying the
axial s ite s , the local metal ion symmetry is a c tu a lly D ^ .
and the chloride anion are designated as weak fie ld , ligands.
suggests th a t the s p littin g s which a ris e due to
compared to IODq.
Both water
This
symmetry are small
I f th is is tru e , the p o ten tial term in the Hamilto­
nian due to the presence o f a tetragonal f i e l d ,
a perturbation o f the octahedral case.
, may be treated as
With th is assumption one can
construct an energy level diagram fo r a complex in O^ symmetry and sub­
sequently apply group th e o re tic a l methods to determine the possible
s p littin g patterns in lower symmetries.
75
Before tra n s itio n s can be assigned, the ground and excited states
must be described in terms o f the possible states fo r the various sym­
m etries of in te r e s t.
The ground s ta te fo r the free metal ions may be
determined according to Hund1s Rule;
in order to fin d a ll possible
Russel I -Saunders(R-S) terms fo r the excited s ta te s , a ta b le o f states
in d ic a tin g the number o f determinants fo r each Ml -Ms combination must
be constructed.
7
As an example, fo r the d
8
or d ions in which two
electrons or two holes, positrons, re s p e c tiv e ly , are placed in the fiv e
o
101
d o rb ita ls with e ith e r o f two spins, there are C10 =
= 45 possible
s ta te s .
With the aid o f an Ml -Ms ta b le these are resolved in to fiv e
energy le v e ls , the R-S terms ^G, ^F, *D,
terms Hund's Rule designates
3
and *S.
F as the ground s ta te .
Of these fiv e
Table X II summar­
izes the number o f states and the term symbols fo r each configuration
of the d o r b ita ls .
The ground s ta te is lis te d f i r s t in each case.
By applying the symmetry operations of any point group to an o r­
b i t a l , one can determine it s representations in th a t p a rtic u la r group.
The d i f f i c u lt y of the ca lc u la tio n depends upon the o rb ita l involved; op­
e ratin g on the d o rb ita ls is a cumbersome task.
Fo rtun ately, there is
a method which allows the representation o f an o rb ita l to be w ritte n
knowing the characters of only the ro ta tio n symmetry operators.
Using
the transform ation m atrix fo r ro ta tio n by an angle, a , the trace is
given by Equation 8 :
X(o) -
I
n=0
e( l- n ) ia
=
.sin (H%)o;
(8)
sin
When a = 0 , which corresponds to the id e n tity operation, the trace is
21+1.
The reducible representation can thus be found in any symmetry
76
Table X II
Russell-Saunders Term fo r Free Ions
Configuration
QOO
CL
PO
d1 (d9)
Term Symbols
Number of
States
10
2D
45
3F, 1G, 1D, 3P, 1S
d3 (d7 )
4 F, 2F, 2H, 2G, 2D, 4 P, 2P
120
d4 <d6)
5D, 3D, 2x1D, 1I , 3H, 3G, 2x 1G,
210
2x 3F, 1F, 2x 3P, 2x 1S
d5
6S, 2S, 2 I , 2H, 4 G5 2x 2G, 4 F 5 2x 2F,
252
4D, Sx2D, 4P, 2P
fo r any o r b ita l by c a lc u la tin g the character of the various rotation s
from Equation 8 .
The states described by the irre d u c ib le representa­
tions are designated g i f they possess a center o f in ve rs io n , u i f they
change sign upon inversion.
The m atrix above is derived from eim^ , where the quantum number, m,
may take on 21 + I values fo r the o r b ita l involved.
S im ila rly , the
wavefunction fo r a R-S term may be expressed as e "*^ , where M may have
2L + I values.
This correspondence enables one to use the symmetry of
the o rb ita ls to define the states in to which the analogous R-S terms are
77
s p lit in various environments.
These representations are tabulated
fo r Oh , D4h and D2h symmetries in Table X I I I .
2
8
3
In the d example^ which is equivalent to d , the. F ground state
o f the fre e ion w ill s p li t in to A2g + B^g + B2g + 2Eg in a tetragonal
f ie ld .
Although the f o rb ita ls are antisym metric, the basis set o f d
o rb ita ls is symmetric;
th e re fo re , regardless o f the spin o r b it coupling
introduced by F, the overall symmetry of the e le c tro n ic wavefunction
must re ta in it s g character fo r any d configuration .
I t is again
pointed out th a t group th e o re tic a l calculatio ns only in d ic a te which en­
ergy le v e ls may be present in a given environment;
these predictions
reveal nothing about the r e la tiv e s p lit t in g .
As a general ru le the R-S terms o f excited states are ranked in en­
ergy according to angular momentum, L, and spin m u lt ip lic it y , 2S + I ;
the greater the value o f L or S, the lower in energy the term.
ever, there are many exceptions to th is ru le .
How­
The r e la tiv e energies
o f the excited states of the fre e ions may be taken from the ordinate
o f a Tanabe-Sugano (T-S) diagram.
The energy states a ris in g from the
s p littin g o f the R-S terms, previously obtained from the character ta ­
bles by group theory methods, may also be taken d ire c tly from T-S d ia ­
grams fo r ions in an octahedral, Oh, f i e l d .
Since both D4h and D2h
symmetries are treated as perturbations o f the octahedral environment of
a metal io n , the weak f ie ld s p littin g o f Oh serves as a reasonable
s ta rtin g point in the determination of the possible tra n s itio n s o f these
lower symmetries.
Reference to appropriate T-S diagrams provides a
valuable piece o f addition al data, the ground sta te in Oh, a re s u lt
unattainable from group theory alone.
I f the ground s ta te in Oh is a
T a b le X III
S p littin g o f Russell Saunders Terms in Various Symmetries
Term
°h
S
P
D
F
G
H
I.
°4h
Aig
%
T lu
A2U +
u
+ Tlu +
T2u
Aig + Eg + T ig + T2g
Eu + 2T l u +
Ag
■ Blu + b2u + 1B3u
=U
flIg + 8Ig + B2g + =g
Eg + T2g
A2
D2h
T 2u
Alg + A2g + =g + T lg + 2T2g
A2u + Blu .+
B2 u
+ 2Eu
2A9 + Blg + B2g - a g
Au + 28Iu + 2B2u
2B3u
2Alg + A2g + Blg + B2g + 2=g
3Ag
Alu + 2A2 u + 6Iu + b2u + 3Eu
2Au + 36IU + 3B2u + 3B3u
2Alg + A2g + 28Ig + 2B2g+3Eg
4V
28Ig + 2B2g + 2B3g
36Ig + 3B2g
3B3g
79
one-dimensional representation , symmetry considerations allow the ground
s ta te to be sp ecified in D4h and D2h also.
Some ambiguity arises i f
the octahedral ground s ta te is m ulti-dim ensional.
For example, a
s ta te s p lits in to an A2g + Eg in D4h, e ith e r o f which may be the new
ground s ta te .
Table XIV l i s t s the ground states possible fo r d^ - d^
configurations.
Table XIV
Ground States in Various Symmetries
Term
?h_
°4h
° 2h
Ai g
Ai g
Ag
T 2g
B2g ° r Eg
Bl g ’ B2g ^
Tig
A2g or Eg
6 Ig - B2g or 8Sg
A2g
Big
B3g
Ag
Observed spectral bands fo r tra n s itio n metal species in the v i s i ­
ble region are generally produced by e le c t r ic dipole tra n s itio n s .
Whether or not a given tra n s itio n is allowed in a free ion can be de­
termined by the p a r ity of the in te g ra l <^g|Ml^e>
o f ij>e and
where the symmetries
are those o f the ground and excited e le c tro n ic s ta te s .
The value of th is integral is defined as the transition moment, and M
is the dipole moment operator.
80
M =
( 9)
Z e.v%
( 10)
where r = x t + y j" +
The symmetry of the operator is th a t o f the ,x,
and z coordinates;
the representations fo r the components o f the operator w ill therefore
always be odd.
Table XV li s t s these representations as found in the
character tables fo r 0h , D4h and D ^ .
If
and ^
are both even
functions, as they must be a ris in g from d o r b ita ls , th is in te g ra l w ill
be odd and w ill vanish.
As mentioned e a r l ie r , d-d tra n s itio n s are
p a rity forbidden.
,
,
Table XV
Representations o f the Dipole Moment Operator
°h
°4h
Tlu (x , y , z)
Eu
■
° 2h
(x , y )
A2u (z )
6Iu <z >
B2u (y)
B3u M
Magnetic dipole tra n s itio n s may also be observedj but are usually
much less intense than e le c tr ic dipole tra n s itio n s .
I f only the e lec­
tro n ic wavefunction is considered, an allowed e le c tr ic dipole tra n s itio n
3
is about 10 times as intense as an allowed magnetic dipole tra n s itio n .
The magnetic dipole moment operator has the symmetry o f the cross
product, r x p, and transforms as the ro ta tio n a l representations, Rx , Ry
or Rz o f the point group in question.
Because these representations
possess g symmetry, magnetic dipole tra n s itio n s are p a rity allowed fo r
81
d-d tra n s itio n s .
The in te g ra l which must be considered to determine
whether the magnetic dipole tra n s itio n s are symmetry allowed is
a
IrI ^ '
The d ire c t product is found in the same way as fo r e le c ­
t r i c dipole tra n s itio n s and must s t i l l contain the t o t a lly symmetric
representation fo r an allowed tra n s itio n to occur.
U tiliz a t io n o f the symmetry o f the e le c tr ic dipole moment operator
to determine whether or not a tra n s itio n is allowed assumes th a t the
e le c tro n ic wayefunction,
is completely independent o f the v ib ra ­
tio n a l wavefunction, ib .
When the ion is in a c ry s ta l, v ib ra tio n of
the atoms may momentarily destroy the g character o f
s ite symmetry.
coupling.
Such in te ra c tio n between \pe and
in the local
is c a lle d vibronic
An expanded in te g ra l can now be w ritte n ,
to describe the tra n s itio n moment.
|M|^v^e> ,
I f th is in teg ra l is nonzero, the
tra n s itio n is v ib ro n ic a lly allowed.
The spectral bands which arise
3
from such tra n s itio n s are on the order o f 10 times weaker than those
which are symmetry and p a r ity allowed fo r pure e le c tro n ic tra n s itio n s .
The introduction o f vibronic coupling implies th a t the d ire c t
product o f the characters o f fiv e representations must be calculated to
determine whether or not the in te g ra l vanishes.
fie d to some e x ten t.
Since
This can be sim p li­
is t o t a lly symmetric in the ground
s ta te , m ultip lyin g by it s representation w ill not change anything, and
i t need not be considered.
Also, any representation m u ltip lie d by i t ­
s e lf must contain the t o t a lly symmetric representation.
Therefore, i f
one of the representations spanned by the d ire c t product o f ^
M \pe Is
also one o f the odd noma I modes df v ib ra tio n o f the excited s ta te , i|/y ,
the in te g ra l w ill be nonzero.
For a ll nonlinear complexes, SN - 6
82
v ib ra tio n a l modes e x is t , where N is the number o f atoms in the complex;
in octahedral coordination N = 7, giving ris e to 15 modes.
Their rep­
resentations are determined by assigning normal coordinates to each
atom and observing how they transform under the symmetry operations of
the point group in question.
Each character is a sum o f 21 terms,
most o f which fo rtu n a te ly turn out to be zero.
the f i r s t operation other than E is C^iz) .
For example, in D2h
I f a MLg molecule is ro ta ­
ted 180° about the z_ a x is , four atoms change p o sitio n s, thus contribu­
tin g nothing to the tra c e .
This ro ta tio n is shown in Figure 28 where
is defined by the M-L^ bond.
"I
X
^
L3
L
■V
Figure 28
Cg Rotation in Normal Coordinates
The three atoms which l i e along the ro ta tio n axis do not move;
coordinates transform according to the follow ing m atrix.
indicates the coordinates a f t e r the ro ta tio n operation.
Xl
V1
?!
- 1 0
Zi
0
0 - 1 0
Z'
0
0
I
th e ir
The prime
83
The coordinate a x is , Z, which is also the ro ta tio n a x is , is unchanged
by the op eration, but X and Y become -X and -Y .
This occurs fo r three
atoms so the trace is -3 fo r the C2 (z ) operation in
This process
is continued fo r each operation in the po int group.
The re su ltin g rep­
resentation is reduced and the terms due to tra n s la tio n and ro ta tio n
are removed.
The remaining terms are the irre d u c ib le representations
o f the normal modes o f v ib ra tio n o f the complex.
fo r
These are tabulated
and D2^ symmetries in Table XVI.
Table XVI
Normal Modes o f V ib ratio n
'h
'4h
lZh
%
+ Eg + 2Tlu + T 2g + T 2u
2Alg + 6Ig + B2g + Eg
+
2A 2 u +
3Ag + Blg + B2g + B3g + 36Iu +
B2 u
3B 2 u +
+ 3Eu
3B 3 u
The tra n s itio n moment in teg ra l w ill also vanish i f the i n i t i a l and
fin a l states do not possess the same spin;
AS = 0.
th is is the selection ru le
The e le c tro n ic wavefunction may be w ritte n as the product of
o rb ita l and spin components, ipe =
•
Since the dipole moment oper­
a to r does not involve spin , the in te g ra l may be w ritte n :
<ifV |iK.>
(11)
Orthogonality requires the second in te g ra l to be zero, i f the spin
wavefunctions o f the excited and ground states d if f e r .
With the symmetry and s p lit t in g inform ation contained in Tables
84
X II through XVI, one can determine whether a p a rtic u la r tra n s itio n is
allowed or forbidden in the geometry described by a given point group.
Ligand coordination fo r the complexes o f the series investigated in
th is work suggests th a t
local s ite s .
is the highest possible symmetry fo r the
Pleochroism and p o la riz a tio n data in d ic a te th a t an even
lower symmetry, D2h, best describes the metal environment.
T-S d ia ­
grams provide the r e la tiv e positions o f energy le v e ls fo r regular octa­
hedral complexes only.
To construct an energy level diagram fo r D2h
complexes, one may e ith e r perform the tedious numerical calculations
required to fin d the sign and magnitude o f the s p lit t in g , or one may as
sume th a t the fu rth e r s p lit t in g o f the octahedral le v e ls in lower sym­
metry is small compared to th a t which occurs going from spherical to
octahedral local s ite symmetry.
Since SCF computations o f open shell
configurations are beyond the scope o f th is in v e s tig a tio n , the la t t e r
assumption is made.
In the complexes in question the tra n s itio n moment operator, T ^
in Oh, indicates th a t a ll the, tra n s itio n s are allowed.
In D4h a ll of
the tra n s itio n s are allowed in x- or ^ -p o la rize d li g h t , w hile some are
forbidden in z-p o la rize d lig h t .
but not pleochroism.
This pattern allows fo r dichroism,
Only in D2h symmetry do the three components of
o f the tra n s itio n moment operator transform as three d iffe r e n t represen
ta tio n s , and thus permit three d iffe r e n t spectra.
A large number of workers have published crystal spectra of the di
valen t ions o f Mn, Fe, Co, Ni and Cu in various environments, but very
little
inform ation c o rre la tin g p o la riz a tio n and pleochroic e ffe c ts ap­
pears in the lit e r a t u r e .
In addition most o f the spectral
85
assignments are made u t iliz in g octahedral or square planar symmetry, an
assumption which does not allow fo r the pleochroism so s trik in g ly e v i­
dent in the mixed metal c rystals o f the TMAiyixM ^ xC l 3 * 2H20 s e ries .
Even when polarized spectra were taken in many cases a sing le crystal
fa c e , th a t produced by the growth h a b it, was u t iliz e d .
C rystals which
grow with faces aligned along e ith e r a v ib ra tio n d ire c tio n or a u n it
c e ll axis are not cpmmon;
spectra remain unresolved.
th e re fo re , many of these polarized crystal
No system reported in the lit e r a t u r e o f­
fe rs the s im p lic ity of orthorhombic symmetry coupled with the wide
v a ria tio n in color demonstrated by the metal ions in th is s e ries .
Energy level diagrams fo r the follow ing spectral discussions are
based on lit e r a t u r e data wherever possible;
in most cases the symmetry
has been lowered and s p littin g assignments have been made to best f i t
the observed polarized spectra.
lowing order:
(4 2 -4 6 ), N i ( I I )
The spectra are discussed in the f o l ­
M n (II) ion (3 1 -3 6 ), C o (II) ion (3 4 -4 1 ), F e ( II ) ion
ion (47-51) and C u (II) ion (5 2 -6 1 ).
86
MANGANESE SPECTRA
The f i r s t spectra to be examined in d e ta il are those o f the pure
.
Mn. crystal TMAMnCl3* 2 ^ 0 .
For a complex containing high spin M n (II)
a ll tra n s itio n s are spin forbidden as w ell as p a rity forbidden, i f only
the e le c tro n ic wavefunction is considered.
The p a rity re s tr ic tio n
may be removed by invoking vibronic coupling, but because these tra n ­
s itio n s are s t i l l spin forbidden, the observed peaks should be narrow
and r e la tiv e ly weak in in te n s ity .
The ground s ta te , ^S in R-S nota­
tio n , is a h a l f - f i l l e d subshell which transforms as the t o t a lly symmet­
r ic representation in any point group.
The tra n s itio n s to excited
states require the pairin g o f two electro ns;
these excited states are
the qu artet terms G, D, P and F lis te d in order of increasing energy.
This ordering is obtained from the T-S diagram.which was constructed by
u t iliz in g SCF methods.
With reference to a T-S diagram and the in fo r ­
mation in Table X I I I an energy level diagram fo r the v is ib le spectra of
weak f ie ld M n (II) complexes in
symmetry may now be constructed.
This is shown in Figure 29.
For high spin M n (II) complexes the ground sta te is Ag in D2^, so
only the characters o f the operator and the excited e le c tro n ic state
need be considered to determine whether or not a given tra n s itio n is
. v ib ro n ic a lly allowed.
gy level fo r M n (II) in
T-S diagrams show th a t the lowest excited ener­
symmetry is T^g which s p lits in to B^g + B2g +
Bgg in D2^1 as shown in Table X I I I .
I f x -p o larize d lig h t is used to
e x cite the complex, the tra n s itio n moment in te g ra ls are <Bxg.lB3UlAg> »
<B2g IB3u lAg> and <B3g lB3u lAg> fo r e le c tr ic dipole tra n s itio n s to these
excited s ta te s .
The d ire c t products are B2u, B^u and Ay resp ec tiv e ly .
87
Because Ay is not one o f the normal modes o f v ib ra tio n in D2h, the l a t ­
t e r tra n s itio n is v ib ro n ic a lly forbidden.
tra n s itio n and v is ib le in
polarized lig h t .
A peak corresponding to th is
or ^ -p o la riz e d lig h t would disappear in x-
Calculations s im ila r to those above have been per­
formed fo r the possible tra n s itio n s in a ll p o la riza tio n s which would
give ris e to peaks in the near IR and v is ib le regions o f the spectra fo r
M n (II) complexes o f D2h symmetry.
The re su lts in d icate one forbidden
tra n s itio n fo r each p o la riz a tio n d ire c tio n :
Ag
■+
Bgg in x-polarized lig h t
Ag -> B2g in y - polarized lig h t
Ag -> Blg in z^polarized lig h t
A ll other tra n s itio n s are v ib ro n ic a lly allowed and should show no po­
la r iz a tio n e ffe c ts .
The problem o f r e la tiv e ordering o f.th e energy le v e ls giving ris e
to these peaks now becomes im portant.
Group theory provides no in fo r ­
mation on the magnitude o f the s p lit t in g and SCF methods have not been
applied in D2h symmetry fo r these complexes.
The energy le vels were
therefo re ranked a r b it r a r i ly so as to y ie ld the best c o rre la tio n of the
observed peaks and th e ir p o la riza tio n s with the predictions of group
theory.
For the spectra of the TMA s a lts which contain M n (II) i t was
necessary to invoke ra th e r la rg e s p littin g s in two cases to account fo r
the experimental observations.
With th is assumption, however, the
M n (II) spectra can be s a tis fa c to r ily explained with the exception of a
very weak band a t 460 nm.
n e tic dipole tr a n s itio n .
This band was u ltim a te ly assigned as a mag­
The tra n s itio n assignments fo r the observed
peaks in order of increasing energy are discussed in. d e ta il below.
88
Figures 5-14 display the polarized spectra o f the Mn containing
c ry s ta ls ;
Figure 5 s p e c ific a lly contains the polarized spectra from
crystals o f the pure s a lt , TMAMnCl3*
.
The follow ing discussion is
i'
lim ite d to those peaks due to the M n (II) ion.
The lowest energy peak
seen consistently in a ll c rystals occurs as a broad band with a maximum
a t 530 nm;
evidence exists in some crys ta ls o f another sm aller peak a t
about 580 nm.
The crys ta ls in which th is peak appears are o f very high
q u a lity and thus i t is trea te d here as a real peak.
In lig h t of v a r i­
ous p o la riza tio n s the band a t 530 nm shows d e fin ite changes in shape,
the most dramatic o f which is a very large decrease in in te n s ity in y polarized lig h t .
In x-p o larize d lig h t th is band is most intense while
the band a t 580 nm has t o t a lly disappeared.
both y - and ^ -p o la riz e d lig h t ;
Two peaks are observed in
th e ir in te n s itie s are greater in ^-po­
la riz e d li g h t , although not as large as the 530 nm peak in ^ -p o la rize d
lig h t .
Inspection o f Figure 5 suggests th a t the r e la tiv e ordering of
these energy le v e ls a ris in g from the octahedral T ^ level should be
Bgg <
< Bgg;
th is best f i t s the observed p o la riz a tio n e ffe c ts as is
shown in the energy level diagram fo r M n (II) in Figure 29 on the f o l ­
lowing page.
I t is lo g ic a l to assign the Ag -* B3g tra n s itio n to the
peak a t 580 nm which disappears in ^ -p o la rize d li g h t , as i t should
based on group th e o re tic a l pred ictio n s.
The band centered around 530
nm can reasonably be B^g + Bgg as i t has maximum in te n s ity in ^ -p o la ­
rize d lig h t where both tra n s itio n s are allowed.
The Ag
Bgg tra n s i­
tio n seems to be stronger than the Ag -> B^g tr a n s itio n , because th is
band, is more intense in ^ -p o la rize d lig h t than in y -p o la riz e d lig h t .
However, due to v a ria tio n s in crystal thickness and sc atte rin g as the
O
h
D2h
Forbidden
Observed
P o la riza tio n
Peaks
Z
T
X
' I
---A n
------T
2g
- " k l
y
none
Z
350 nm
y
367 nm
X
X
'---A S
:
330. nm
none
412 nm
418 nm
Z
y
460 nm
y
530 nm
X
,580 nm
Z
CO
CO
CQ
Z
/
Z
A
ig
Figure 29
A
g
M n (II) Spectral Transitions
90
crystal is rotated to obtain a ll three p o la riz a tio n d ire c tio n s , in te n ­
s ity is not very conclusive evidence fo r the la t t e r statement.
The peak a t 460 nm cannot be assigned s o le ly as an e le c tr ic dipole
tr a n s itio n .
I t is v is ib le in y - polarized lig h t , disappears in ^ -p o la ­
rize d lig h t and is barely v is ib le in ^ -p o la riz e d lig h t .
There are two
tra n s itio n s which should occur in th is region, Ag -> Blg and Ag -+ B^g ;
the former is forbidden in ^z-polarized fo r an e le c tr ic dipole tra n s i­
tio n , allowed fo r a magnetic dipole tr a n s itio n .
Since no peak appears
in z - polarized li g h t , the weak peak a t 460 nm v is ib le in x-p o larized
lig h t is assigned as the Ag -> Blg e le c tr ic dipole tr a n s itio n .
This
extremely weak tra n s itio n is lo s t under the la rg e r peak present in y polarized lig h t .
The behavior of th is la rg e r peak suggests th at
Ag -+ Bgg may be a magnetic dipole tr a n s itio n .
The operator symmetry
fo r these tra n s itio n s is even and magnetic dipole tra n s itio n s are a l ­
lowed in ju s t those p o la riza tio n s in which e le c tr ic dipole tran s itio n s
are forbidden.
This im plies th a t Ag -> Bgg is allowed only in y -p o la -
rize d lig h t which f i t s the observed spectra. .
Two d is tin c t peaks which behave q u ite d iffe r e n tly in polarized
lig h t occur a t 412 nm and 418 nm.
In ^ -p o la rize d lig h t the very sharp
peak a t 412 nm disappears completely;
the broader, less intense peak
a t 418 nm displays no d e fin itiv e change in any p o la riz a tio n .
If a
large s p lit t in g of the Tgg(^G) energy level occurs, the tra n s itio n
Ag -* Bgg could be the source o f the peak a t 412 nm;
th is leaves the
Ag -> Bgg tra n s itio n a t lower energy which corresponds with the peak a t
460 nm.
The three Ag le vels a ris in g from Eg and Al g (^G) could y ie ld
the unpolarized peak a t 418 nm, and th is peak is broad enough to
91
suggest th a t there may be more than one tra n s itio n under the envelope.
A s im ila r large s p lit t in g o f the Tgg(^D) is required to account
fo r the p o la riz a tio n o f the peaks a t 367 nm and 350 nm.
Again the A
9
le v e ls from Eg(^D) should give ris e to an unpolarized peak9 but super­
imposed on th is peak a t 350 nm is the Ag -*■
tra n s itio n . . This peak
is d r a s tic a lly reduced in in te n s ity in z-p o la rize d li g h t , but does not
completely disappear, presumably because o f the presence o f the unpola­
rize d Ag -*■ Ag tra n s itio n s .
At about 330 nm the highest energy peak in the spectral region is
recorded.
The s l i t on the Cary 14 reaches maximum aperture fo r useful
spectra in th is region;
strument a r t i f a c t .
thus., th is peak could conceivably be an in ­
However, although not v is ib le in a ll c ry s ta ls , the
peak is consistently a t 330 nm when i t is seen;
^ -p o la rize d lig h t .
i t also disappears in
I f i t is indeed a v ia b le tr a n s itio n , i t may be as-
signed as the Ag -> B^g tra n s itio n a ris in g from the octahedral Tlg ( P ) .
Foster and G ill (31) have published unpolarized crystal spectra
fo r several M n (II) s a lts based on octahedral geometry.
Although none
of the c rystals studied by these authors possess the same chromophore
as TMAMnCl2»ZHgO, the energies reported fo r the observed peaks in the
v is ib le region agree q u ite well w ith those found in th is work.
D ingle,
e t. al_., (33) have examined the po larized crystal spectra a t low tem­
perature of [ (CH3)^N]MnClg;
th is hexagonal s a lt e x h ib its .tw o d iffe r e n t
spectra as expected fo r a un iaxial c ry s ta l.
crystal spectra of MnClg«ZHgO.
same chromophore as the TMA s a lt .
Lawson (35) has reported
This monoclinic compound contains the
Because the tra n s itio n s fo r the
M n (II) ion are spin forbidden, varying the ligand and geometry does not
92
re s u lt in a very large energy s h if t fo r the observed peaks.
makes c o rre la tio n o f the spectra a straightforw ard task.
This
Table XVII
li s t s the MnClg^HgO peaks along with those obtained in th is work and
also assigns p o la riza tio n s to the tra n s itio n s .
Table XVII
Location and P o la riza tio n o f M n (II) Peaks
MnCl2- 2H20
TMAMnCl3- 2H20
Forbidden
Peaks (nm)
Peaks (nm)
P o la riza tio n
325
330
y?
348
350
z
367
373
412
367
X, y
412
x
416
418
none
466
460
x, z
525
530
y. z
580
X
To account fo r the observed p o la riz a tio n of peaks in the TMA s a lt ,
a s p littin g o f the T2g(^G) energy level large enough to cause overlap
w ith the Eg(^G) s ta te had to be invoked.
be in accordante w ith the lit e r a t u r e data.
This assumption appears to
The major peaks o f the
spectra observed in th is work are in e x c e lle n t agreement w ith those of
Lawson recorded fo r the same chromophore.
This close agreement
93
im plies th a t the o rig in a l assumption o f a small perturbation o f the
energy le vels in octahedral symmetry to account fo r the s p littin g in
D2J1 symmetry is a v a lid one.
Although there are more possible tra n ­
s itio n s due to the decrease in symmetry, the s p lit t in g is small and
broader peaks are usually seen ra th e r than more peaks.
In one in ­
stance a sharp new peak is introduced (th a t a t 412 nm) which is a sin ­
g le , nondegenerate polarized tra n s itio n ;
G ill has no evidence of th is
p a rtic u la r tra n s itio n fo r the unpolarized spectra in octahedral sym­
metry.
On the whole the predictions o f group theory are borne out
q u ite well by the observed polarized crystal spectra o f the M n (II) ion.
94
COBALT SPECTRA
Of the f i r s t row tra n s itio n metals u t iliz e d in th is work, M n (II)
is the only ion fo r which the tra n s itio n s are spin forbidden as well as
p a rity forbidden;
in the spectra o f the Fe, Co, Ni and Cu s a lts the
observed tra n s itio n s are spin allowed.
The peaks fo r these ions are
thus broader and much more intense than those fo r M n ( II ) .
In addition
the near IR region must be scanned fo r these ions, because some o f the
lower energy peaks are seen there ra th e r than in the v is ib le region.
In order to keep the peaks on scale fo r spectra o f c rystals containing
the d® through d^ ions, e ith e r very th in crystals or crys ta ls with a
very low mole fra c tio n o f these ions had to be selected.
By grinding
an extremely th in cross section o f a crystal o f TMACoC I3* 2H20 , p o larized
spectra were obtained fo r th is pure s a lt in the same manner as fo r the
Mn analog.
The Mn-Co mixed crystals provided a much less intense
spectrum fo r the C o (II) io n , although i t was complicated by the pres­
ence of the M n (II) ion.
The higher energy peaks o f the Mn spectrum
are s t i l l v is ib le and v ir t u a lly unaffected, but the two lowest energy
tra n s itio n s are lo s t beneath the much la rg e r peaks due to the Co ion.
Comparison of the spectra of the Mn, Mn-Co and Co analogs in d i­
cates th a t there is no in te ra c tio n between the metal ions in these
c ry s ta ls ;
the Mn and Co individual spectra are simply a d d itiv e .
There is a marked loss o f in te n s ity in Mn and Co in divid ual peaks in the
spectra o f the mixed metal c ry s ta ls , but th is is to be expected due to
lower metal concentrations fo r both ions.
The habit fo r the. mixed
metal c rystals also made thinner crys ta ls a v a ila b le fo r these spectra.
N evertheless, there is a good mapping o f the Co peaks in the pure s a lt
95
to those found in the mixed c ry s ta l, i f one ignores in te n s ity d iffe r e n C
ces which cannot be avoided.
The peaks due to the d
ion also map,
except fo r the two which are lo s t beneath the intense tra n s itio n s of
the Co ion.
Because o f the in te n s ity differences between a spin a l ­
lowed and spin forbidden tr a n s itio n , i t is assumed th a t any p o la riz a ­
tio n e ffe c ts fo r the peaks in th is region are due s o le ly to the C o (II)
ion.
The R-S ground s ta te fo r C o ( I I ) 9 a d
io n , is
F;
in an octahe­
dral f ie ld th is s p lits in to A2g +■ T2g + T1 , the la t t e r o f which is the
ground s ta te as seen in T-S diagrams.
When the degeneracy of the Tlg
level is removed by lowering the symmetry to D ^ , . three new energy
le vels a r is e , Blg + B2g + Bgg , any o f which could be the new ground
s ta te .
Without access to SCF methods the actual ground s ta te may be
assigned by c o rre la tin g the predicted tra n s itio n s with the observed
p o la riza tio n data;
fo r each ground s ta te nine spin allowed tra n s itio n s
which may y ie ld peaks in the v is ib le or near IR regions must be con­
sidered.
Group th e o re tic a l evaluation o f the tra n s itio n moment in te ­
grals and comparison with the polarized spectra in Figures 6-9 e lim in ­
ate B2g as the ground s ta te .
Both Blg and Bgg f i t portions of the
C o (II) spectra, but n e ith e r is s a tis fa c to ry fo r the e n tire IR -v is ib le
range.
Because id e n tific a tio n o f the ground s ta te is cru c ia l to the
discussion o f the TMACoClg'2H20 spectra, previously published spectra
o f other C o (II) crys ta ls were examined to establish a v a lid framework
upon which to base tra n s itio n assignments.
Several a r tic le s appear in the lit e r a t u r e (34-41) which contain
polarized crystal spectra o f the C o (II) ion in an environment s im ila r to
96
th a t found in the TMA s a lt .
Ferguson and Wood have published low
temperature data fo r CoCl2-SH2O (38) and CoCl2-EH2O (3 9 ).
The d ihy­
drate is o f p a rtic u la r in te r e s t, because the chromophore in th is com­
pound, CoCl^(H2O)2 , is the same as th a t found in the TMA s a lt .
energy level diagram has been constructed fo r
An
symmetry based on the
data fo r the hexahydrate which contains Co(H2O)^Cl2 octahedra.
How­
ever, n e ith e r the spectra o f CoCl2-EH2O nor TMACoClg-EHgO allows a rea­
sonable in te rp re ta tio n o f the p o la riz a tio n patterns in tetragonal sym­
metry.
Ferguson and Wood have attempted to explain the IR spectra of
the d ihydrate as magnetic dipole tra n s itio n s in C2h symmetry.
Choice
o f ground s ta te is again ambiguous, and regardless o f the choice made,
a ll o f the observed peaks cannot be assigned.
For the TMA s a lts the e le c tr ic dipole moment operator was o rig in ­
a lly chosen to describe the spectra because most peaks persisted in two
p o la riza tio n s ;
such behavior is ty p ic a l o f d-d vibronic tra n s itio n s .
In the Mn s a lt only one peak appeared which could not be assigned as an
e le c tr ic dipole tra n s itio n .
In the Co analog many o f the peaks de­
crease in in te n s ity in two p o la riz a tio n s , behavior in d ic a tiv e of mag­
n e tic dipole tra n s itio n s .
Another problem encountered in dealing with
the C o (II) spectra is th a t not a ll o f the possible tra n s itio n s are seen
in the TMA s a lt , making assignment o f peaks a te n ta tiv e process.
Because n e ith e r e le c tr ic nor magnetic dipole tra n s itio n s alone can
account fo r the C o (II) spectra of these d ihydrates, the question as to
whether both could be present must be raised .
The in te n s ity of a sym­
metry allowed magnetic dipole tra n s itio n is much weaker than th a t o f a
symmetry allowed e le c tr ic dipole tr a n s itio n .
However, in the solid
97
s ta te where thermal vib ratio n s are much weaker, one,can imagine th a t
an e le c tr ic dipole tra n s itio n which is p a rity forbidden and can be ex­
plained only by vibronic coupling, might a c tu a lly be of the same order
o f in te n s ity as the p a rity and symmetry allowed magnetic dipole tr a n s i­
tio n .
Thus, in these crystals one might reasonably expect a magnetic
dipole tra n s itio n to appear a t room temperature and an e le c tr ic dipole
tra n s itio n to p e rs is t a t very low temperature.
Work done in th is lab
a t room temperature on several c rystals containing C o (II) has yielded
consistent spectra with w e ll-d e fin e d peaks in the v is ib le region, but
good IR spectra have proven d i f f i c u l t to a tta in .
Lacking low tempera­
tu re data fo r the TMA s a lts , there is no evidence to distinguish be­
tween the two types o f spectra in th is work.
The argument presented above can be tested on spectra taken from
■
the lit e r a t u r e such as those reported by Ferguson and Wood (3 8 ,3 9 ).
The energy level diagram fo r C o (II) in
symmetry is presented in
Figure 30 along with possible s p littin g patterns i f the symmetry is
lowered to D2^.
Also included in th is fig u re are the locations of
peaks in the CoC12*2H20 spectra and the p o la riza tio n s calculated assum­
ing th a t e le c t r ic dipole tra n s itio n s are involved.
The r e la tiv e o r­
dering o f the Dz^1 energy le v e ls , which establishes A2g as the ground
s ta te , is based on the spectral data o f Ferguson and Wood fo r the hexahydrate, CoCl2- SH2O.
Explanation o f the spectra of the d ihydrate,
CoCl2eEH2O5 requires a lower symmetry.
The spectral measurements fo r
the l a t t e r s a lt were made on c rystals which were o rie n te d .w ith orthog­
onal axes, a / , b_ and £ where a' and j) were defined by e x tin c tio n d ire c ­
tio n s .
These axes correspond to £ , £ and Ib re sp ectively in the
98
Forbidden
P o la riza tio n
Calculated
R-S
- -B1
' - * 29-
C o C l 2H?0
Peaks^
( R e f .39 )
Forbidden
P o larizatio n
Observed
395 nm CT
y .z
T --------- '
Tig<
--B ,
' - Eg <
-B,
z
505 nm
z
x
610 nm
x,y
y
830 nm
y,x?
none
1200 nm
none
1630 nm
z
none
1
1
1
f
1
1
1
1
\
\
\
\
\
\
z
\
\
\
\
\
Figure 30
Possible Energy Level Diagram fo r C o (II)
99
orthorhombic TMA s a lt .
Careful comparison o f actual p o la riz a tio n d i­
rections with respect to the Co octahedra indicates th a t, although
CoClg'ZHgO is m onoclinic, the v is ib le spectra in a ll p o la riza tio n s a t
IO S are very s im ila r to those in TMACoClg'ZHgO a t room temperature;
these compounds possess the same chromophore.
The highest energy peak
in the near IR also ex h ib its the same behavior in both s a lts .
Such
agreement demonstrates the v a lid it y of the assumption th a t the spectra
o f these two hydrates a ris e from the same energy level diagram.
The
near IR spectra obtained by Ferguson and Wood cannot be s a tis fa c to r ily
explained u t iliz in g magnetic dipole tra n s itio n s in Cg^ symmetry;
in x-
and y - polarized lig h t two peaks are always v is ib le in the observed
spectra whereas only one is allowed based on group th e o re tic a l predic­
tio n s .
In the IR spectra a ll o f the peaks can be explained i f they are
assigned as e le c tr ic dipole tra n s itio n s u t iliz in g
s ta te in
symmetry.
as the ground
U nfortunately, th is requires a ra th e r a rb i­
tr a r y s p lit t in g of the Eg energy level which arises from the T2g( 4F)
term as can be seen in Figure 30.
The tra n s itio n s from B0
to B1
and
B2g which a ris e from the same T^g term occur a t such low energy th a t
they are not seen in the near IR region.
The lowest energy peak ap­
pears a t 1630 nrh in Ferguson's spectra and may be assigned as the tra n ­
s itio n Bgg -> B2g which disappears in z-p o la rize d lig h t .
The peak a t
1200 nm which shows no p o la riz a tio n e ffe c ts is assigned as the Bgg ->Bgg
tr a n s itio n .
The remaining peak a t 830 nm corresponds to the Bgg -» Blg
tra n s itio n which is forbidden in y -p o la riz e d lig h t , but th is is an ex­
tremely weak tra n s itio n in the CoCl2-EH2O spectra.
In addition to the
100
un satisfactory s p lit t in g o f the Eg and B2g levels from D4h$ th is model
suffers due to the to ta l in a p p lic a b ility o f Bgg as the ground sta te fo r
any of the peaks in the v is ib le region.
Selection o f B^g as the ground s ta te in
y ie ld s a more reason­
able s p lit t in g arrangement as shown in Figure 31, but leaves the peak
a t 1630 nm which should be the B^g -> B2g tra n s itio n unexplained e ith e r
as a magnetic or e le c tr ic dipole tr a n s itio n .
The B^g -> B^g tra n s itio n
a t 1200 nm is allowed in a ll p o la riza tio n s as an e le c tr ic dipole tra n ­
s itio n ;
i t is forbidden in a ll p o la riza tio n s as a magnetic dipole
tr a n s itio n .
The peak a t 830 nm can be assigned as the, Blg
Bgg e le c ­
t r i c dipole tra n s itio n which is forbidden in ^ -p o la rize d lig h t .
It
can also be assigned as a magnetic dipole tr a n s itio n , i f one assumes
th a t th is peak has disappeared in the x-p o larize d spectrum.
There is
some ju s t if ic a tio n fo r both o f these arguments.
This second model has another advantage in th a t the Blg ground
state.can be used to in te rp re t the p o la riz a tio n patterns of the peaks
in the v is ib le region as w e ll.
I f s p lit t in g is small as symmetry is
lowered from D4^ to D2^, one would expect Blg as the new ground s ta te ,
because A2g transforms as Bl g .
Although th is is conjecture since the
Eg and A2g energy le v e ls from Tlg (^F) cannot be lo cated , i t lends some
addition al c r e d ib ilit y to the assignment o f Blg as the ground s ta te .
The disadvantage o f the energy level diagram presented in Figure
31 is th a t the peaks in the spectra o f the TMA s a lt must be described
as magnetic dipole tra n s itio n s .
Explanation o f the v is ib le spectra
also requires a tr a n s itio n , a broad band between 500 and 550 nm, which
is very intense, in x - and ^ -p o la rize d l i g h t , but weak in z-p o la rized
101
°4h
4P—
CO
h-i
/'V
/
D2h
CD
°h
I
I
R"S
Forbidden
TMACoCly ZH9O Forbidden
P o la riza tio n
Peaks'^
P o larizatio n
Calculated
Observed
none
425 nm
?
X
515 nm
y sz
y
545 nm
X sZ
Z
610 nm
x,y
y
830 nm
X 5Z
- T1 9 V
X
CO
I
/
CD
CO
; - A2g—
/
g
/
X-E < - ' ' - B2S
"Big " " "Ag
I
I
Z
I
CD
CO
CO
I
I
x---vcr-i,
\
x
\
\
\
\
Figure 31
Preferred Energy Level Diagram fo r C o (II)
102
lig h t .
band;
Ferguson and Wood have designated th is peak as an "anomalous"
i t w ill be discussed separately below.
On top o f th is broad band are two peaks which are assigned as mag­
n e tic dipole tra n s itio n s ;
the Blg -> B2g and Blg
these two peaks a t 515 and 545 nm represent
Bgg tr a n s itio n s , re sp ec tiv e ly .
515 nm is allowed only in ^ -p o la riz e d lig h t ;
The peak a t
th a t a t 545 nm is allowed
only in y -p o la riz e d lig h t .
1
The peak a t 610 nm is the lowest energy tra n s itio n in the v is ib le
region;
i t is assigned as the Blg -» Ag tra n s itio n allowed only in z-
polarized lig h t .
This peak is located a t the same wavelength and is
polarized in the same way in both TMACoClg-ZHgO and CoCl2- 2H20 ;
the
la t t e r displays b e tte r reso lutio n due to lower temperature.
The highest energy peak in the near IR occurs a t 830 nm fo r both
of these complexes as well and also behaves s im ila r ly in polarized
lig h t .
lig h t ;
s itio n .
In the TMACoClg-ZHgO th is peak is seen only in ^ p o la r iz e d
th is corresponds to the Blg
Bgg magnetic dipole moment tra n ­
The remaining two IR peaks were never seen in the spectra of
the TMA s a lt which were taken a t room temperature.
The highest energy spin allowed magnetic dipole tr a n s itio n ,
Blg -> Blg (^P) is forbidden in a ll p o la riz a tio n s ;
appear in any o f the C o (II) spectra.
such a peak does not
Ferguson and Wood report a
charge tra n s fe r band a t 395 nm which increases in in te n s ity as tempera­
ture decreases;
i t is polarized perpendicular to the chain d ire c tio n .
This p a rtic u la r peak is not seen in the spectra of the TMA s a lt , but
absorption is increasing as the s l i t opens beyond useful aperture in
th is region.
A very weak peak appears a t 425 nm in the TMACoClg-ZHgO
103
spectra which is not seen in the CoClg-ZHgO spectra.
Although i t is
most intense in ^ -p o la rize d lig h t , i t is v is ib le in x - and ^ -p o la rize d
lig h t as w e ll.
This is the only peak which is not c le a rly v is ib le in
both the pure Co and the mixed Mn-Co spectra.
Because o f it s low in ­
te n s ity in the pure s a lt , it s absence in the mixed Mn-Co crystal would
not be s u rp risin g .
Nevertheless, i f one examines the Mn-Co spectra
c a r e fu lly , there is some evidence o f a shoulder on the M n (II) 418 nm
peak which may be th is tra n s itio n .
ways:
This peak may be explained in two
as the
-> B ^ e le c tr ic dipole tra n s itio n or as a spin fo rb id 2
den tra n s itio n to one o f the energy le v e ls a ris in g from the G term
which f a l l s in th is range as seen on the T-S diagrams.
is very low fo r an e le c tr ic dipole tr a n s itio n ;
The in te n s ity
thus, the la t t e r in t e r ­
p re ta tio n is p re fe rre d .
Description o f the C o (II) spectra in the TMA s a lt as magnetic d i­
pole tra n s itio n s leaves only one peak unexplained.
This broad, in ­
tense band present in x- and ^ -p o la rize d lig h t almost completely disap­
pears in ^ -p o la rize d lig h t .
Although one would expect i t to be an
e le c tr ic dipole tr a n s itio n , since i t is present in two p o la riz a tio n s ,
no choice of ground s ta te in e ith e r Dg^ or
it s p o la riz a tio n .
symmetry can account fo r
Ferguson and Wood have described th is peak as an
"anomalous" crystal f ie ld band u t iliz in g water vib ratio n s to induce in ­
te n s ity fo r some o f the doublet terms which are spin forbidden.
Be­
cause o f it s in te n s ity , i t is d i f f i c u l t to accept th is peak as a spin
forbidden tr a n s itio n .
The peak is w ell documented, appearing in a ll
C o (II) spectra, but only in symmetry as low as Dgh does it s in te rp re ta tio n become a problem.
I t has always been described as the T ^ ( F) to
104
4
T ig( P) tra n s itio n in octahedral geometry or the A2g to Eg e le c tr ic d i­
pole tra n s itio n in tetragonal geometry;
without p o la riz a tio n data no
c o n flic t arises in terms o f the energy level diagram.
More spectral
work needs to be done u t iliz in g polarized lig h t in complexes of lower
symmetry in order to f u l ly explain the o rig in of th is peak.
105
IRON SPECTRA
Crystals of the pure Fe and the pure Ni s a lts were never grown
large enough fo r spectra to be recorded from them.
However, preces­
sion photographs o f these needle-1ike crystals in d ic a te th a t they are
also orthorhombic, isomorphic with the Mn analog.
This inform ation,
coupled with the fa c t th a t the Mn-Co mixed metal spectra are simply the
sum o f the spectra of the in divid ual pure s a lts , leads one to assume
th a t the F e ( II) and N i ( I I ) spectra may also be extracted from the
spectra o f the corresponding mixed metal analogs.
Crystals o f the
mixed Mn-Fe and Mn-Ni s a lts which are s u ita b le fo r c o lle c tio n of spec­
t r a l data are obtained f a i r l y re a d ily .
Even though spin allowed tra n ­
s itio n s e x is t fo r these two ions, t h e ir spectra should be r e la tiv e ly
simple, because there are fewer possible tra n s itio n s .
The energy level diagram fo r F e ( II) is presented in Figure 32.
5
The R-S ground s ta te , D, s p lits in to Eg + T2g in octahedral symmetry;
the T2g level is the
ground s ta te .
As in the case o f the C o (II)
io n , there is some ambiguity in designation o f the ground s ta te when
the symmetry is lowered to D2^;
any of the three le v e ls which a rise
from T2g may be the D2^ ground s ta te .
The number of peaks observed in
the polarized spectra of the F e ( II) s a lt depends a great deal upon the
magnitude of the s p lit t in g o f the energy le v e ls in orthorhombic symmet­
ry !
I f the s p lit t in g is sm all, only one peak may be v is ib le .
I f the
s p littin g o f the energy le v e ls from Eg is la rg e , one would expect two
peaks, both ground s ta te to Ag tra n s itio n s .
I f the s p lit t in g of the
T2g level is also la rg e , one could expect a maximum o f four spin a l ­
lowed tra n s itio n s in the F e ( II) spectra.
As in the C o (II) spectra.
D2h
(a)
R-3
Oh
/
D4h
(b)
(c )
T ransitio n
T ran sitio n
Transition
z-forbidden
y - forbidden
x-forbidden
, - E V - aI S - - - - /
"
B1 ---------- '
/
ig
Ag
\
Ag
Ag
Ag
Ag
\
639
5D V
\
x-E - ' C
'-Vv
9
"B2g
Figure 32
'-'V
8Ig
... B3g
B2g
Bi g
8Ig
- B2g
B3g
Possible Energy Level Diagrams fo r F e ( II)
Observed
peaks
1060 nm
107
the peaks due to tra n s itio n s among the energy levels from s p littin g of
the Tgg term are not considered, as they are so low in energy th at they
are u n lik e ly to be observed.
considered:
Bgg and Bgg .
Three possible ground states must be
One or perhaps two peaks ought to
occur in the near IR.
A cleavage fragment, a c le a r crystal containing the b-c cross sec­
tio n , was u t iliz e d to c o lle c t the observed polarized spectra of the
Mn-Fe s a lt .
No peaks appear in the v is ib le region other than those
ascribed to the M n (II) ion.
One large peak appears in the near IR a t
1060 nm and is id e n tic a l in shape and in in te n s ity in both y - and
polarized lig h t .
Zr
U nfortunately, due to the habit o f the Mn-Fe c r y s ta l,
the spectrum in ^ -p o la rize d lig h t could not be obtained.
Based on the
evidence a v a ila b le , the assignment fo r th is peak im plies a Bgg ground
s ta te fo r the F e ( II ) ion in Dg^ symmetry.
However, lacking the x-po­
la riz e d spectrum, th is assumption cannot be v e rifie d .
The o p tic a l and near IR spectrum fo r Fe(C^O4 ) (HgO)g in unpolarized
lig h t has been determined by Long (4 2 );
no peaks show in the v is ib le
region and two broad bands appear a t 930 and 1180 nm.
Long assumes
tetragonal symmetry and assigns these as tra n s itio n s from an undeter­
mined ground s ta te to A^g and B^g re s p e c tiv e ly .
tween these two peaks is not good;
The resolution be­
i f they were not resolved, the max­
imum would occur a t 1055 nm which is very close to the value observed
fo r the sing le peak in the TMA s a lt .
I t may be th a t the two energy
levels are not s p li t very much in the TMA s a lt , and thus remain unre­
solved.
Putnik, eix al_., (43) have examined polarized crystal spectra of
108
RbFeClg and CsFeClg a t 4 .2 K.
These compounds c r y s ta lliz e in the
P6g/mmc space group and e x h ib it two colors in polarized li g h t , red and
yellow .
However, the broad band a t about 1330 nm which these authors
id e n tify as the T2g ^ Eg tra n s itio n remains unchanged in a- or ^ -p o la ­
rize d lig h t .
The colpr change in th is compound appears to be due to
p o la riza tio n o f the spin forbidden tra n s itio n s to t r i p l e t s ta te s .
It
is in te re s tin g to note th a t th is compound has hexagonal symmetry as did
the [(CHg)^NjMnClg discussed e a r lie r (33) and is therefo re u n ia x ia l;
two d iffe r e n t spectra are again predicted.
Polarized spectra o f minerals which contain F e ( II) such as g ille s p it e , fa y a lit e and almandine also show a peak in the near IR a t about
1000 nm (4 4 ).
Other workers have found s im ila r peaks (4 5 ,4 6 ).
Thus
the observed spectra o f the F e ( II) TMA s a lt corresponds to those found
in the lit e r a t u r e fo r F e ( II) compounds.
U nfortunately, without the
x-p o larize d spectrum, the ground s ta te cannot be proven to be Bgg.
109
NICKEL SPECTRA
The tra n s itio n s involved fo r the N i ( I I ) ion are illu s tr a te d in
Figure 33.
The ground s ta te in
symmetry, A2g, is already a one­
dimensional representation so in th is case there is no
ing the ground s ta te in
symmetry;
problem assign­
A2g transforms as Ag .
The two
t r i p l e t s , T^g and J 2 , which a ris e from the 3F term are each s p lit in to
Blg + B2g + B3g in any order;
the lower energy term, T2g, probably
involves a tra n s itio n low enough in energy to be located in the near
IR.
There is also a T1 in octahedral symmetry which arises from a
3
P term; th is y ie ld s a th ird band composed o f Blg + B2g + B3g.
The
l a t t e r is the highest energy spin allowed tra n s itio n and should appear
in the v is ib le region.
The Mn-Ni c rystals large enough to be u t iliz e d fo r spectral meas­
urements contained very low mole frac tio n s o f N i ( I I ) ;
le a s t soluble in the M n (II) la t t ic e s it e .
th is ion is the
The z-p o la rize d spectrum
taken on a th in crystal y ie ld s peaks o f extremely low in te n s ity .
Attempts to increase the in te n s ity o f the observed peaks by using l a r ­
ger crystals resulted in so much s c a tte r th a t peaks could no longer be
id e n tifie d .
In the Mn-Ni polarized spectra a broad band appears in the near IR
a t 825 nm which is v is ib le in ^ -p o la rize d lig h t .
I f i t is a c tu a lly
present in x-p o larize d li g h t , i t is a very weak tr a n s itio n .
I t does
not show in ^ -p o la rize d lig h t , but th is may be due to the thinness o f
the crystal and low N i ( I I ) concentration ra th e r than to p o la riza tio n
e ffe c ts .
In the v is ib le region there is a peak a t 440 nm;
th is is
superimposed on the M n (II) 460 nm peak which makes sorting out the
HO
Forbidden
R-S
°h
D2h
, , ' ' - S
T ---------- --- " " - f Igt" -
P o larizatio n
Peaks
y
"Blg
Z
' " B3g
X
440 nm
I
C7>
CO
CO
>
V
\
X
\
>
/ - V c - - S 2g
y
825 nm
Z
/
'" - S
/
/
/
S
y
Z
\
\
\
X
X
X
Figure 33
I
>
CO
X
I
.
CD
CO
CO
X
X
-
v
X
Energy Level Diagram fo r N i ( I I )
I
Ill
p o la riza tio n e ffe c ts d i f f i c u l t .
As the p o la riz a tio n d ire c tio n is
changed, th is peak does indeed s h if t location and change in shape
s lig h tly .
This indicates th a t i t could be the high energy v is ib le
tra n s itio n predicted by group theory, the e le c tr ic dipole tra n s itio n
from Ag to the three closely spaced le v e ls , Bjg + B2g + B^g , a ris in g
from T j ( P ) .
Examination o f the p o la riz a tio n e ffe c ts suggests the
s p littin g B3g lower than B^g lower than B2g. .
The lit e r a t u r e on N i ( I I ) s a lts deals p rim a rily w ith low tempera­
ture in vestig atio n s o f magnetic chains o f the type AMX3 .
The N i ( I I )
compounds RbNiCl3 , CsNiCl3 and [(CH 3 )^N lN iC l 3 (47,49) belong to the
same general category as the F e ( II) compounds (43) which were discussed
in the preceding section.
The N i( I I )
s a lts KMgCl3 (48) and CsMgCl3 (4 9 ).
ion can also be doped into the
A ll o f the above compounds belong
to the PG3Zmmc space group except [(CH 3 )^N lN iC l 3 which c r y s ta lliz e s in
PG3Zm.
Such crystals are uniaxial with the p o s s ib ility o f dichroism.
These workers have obtained polarized spectra;
however, they again de­
scribe the tra n s itio n s in terms o f octahedral symmetry.
Many peaks are
present in the spectra o f these compounds which are not seen in the TMA
s a lt;
spin forbidden as well as spin allowed tra n s itio n s appear in the
low temperature spectra.
The spin allowed tra n s itio n s from A2g to
T jg( 3F) and T j g( 3P) are located at. 1430 nm, 850 nm and 460 nm,
re sp ec tiv e ly .
The lo catio n o f the two higher energy peaks correlates
q u ite well with those observed in the Mn-Ni TMA s a lt a t 825 nm and 440 .
nm.
Resolution of the 440 nm peak allows assignment of the r e la tiv e
order of the B jg + B2g + B3g le v e ls and it s location is supported by
the lit e r a t u r e data.
However, the peak in the near IR a t 825 nm cannot
112
be s p li t based on the p o la riz a tio n data with any degree o f c e rta in ty .
The lowest energy spin allowed tra n s itio n is not seen a t a ll in the
spectra o f the TMA s a lt taken a t room temperature.
113
COPPER SPECTRA
Several serious problems were encountered try in g to obtain crystal
spectra fo r the C u (II) ion.
Due to the Jah n -T eller d is to rtio n exhib­
ite d by the d^ io n , the pure Cu s a lt c r y s ta lliz e s in a monoclinic
ra th e r than an orthorhombic space group.
This makes o rie n ta tio n of
the p rin cip a l v ib ra tio n directions, a more d i f f i c u l t problem than fo r
the orthorhombic c ry s ta ls .
As has been mentioned previously, the u n it
c e ll axes in the pure Cu s a lt are redefined as compared to the ortho­
rhombic u n it c e ll;
by c rystallo g rap h ic convention the unique axis in
the monoclinic c e ll is defined as lb.
In the Cu s a lt th is j) axis hap­
pens to correspond to the d ire c tio n o f hydrogen bonding;
axis is along ja.
the chain
The a-b cross section o f the pure Cu u n it c e ll
th erefo re corresponds to the orthorhombic b-c cross section.
In the
9
2 2
2
d ion the x -y and z o rb ita ls s p li t so th a t the lone electron occu­
pies the higher energy o r b it a l.
T h is , the J a h n -T eller e f f e c t , causes
d is to rtio n o f the square planar CuCl^- arrangement with two o f the
Cu-Cl bonds lengthening, two shortening.
The overall re s u lt is th at
the £ axis of the u n it c e ll is no longer perpendicular to the needle
a x is , £ ;
the c e ll is now monoclinic.
However, even though the u n it
c e ll axes are no longer orthogonal, the three p rin cip a l v ib ra tio n d i­
rections remain so.
The unique axis may define one v ib ra tio n a l d ire c ­
tio n w ithin the monoclinic in d ic a tr ix , but there are no re s tric tio n s
on the lo catio n o f the remaining two.
Location of these v ib ra tio n d i­
rections w ith respect to the u n it c e ll axes is necessary i f unambiguous
spectra are desired.
Examination o f a TMACuClg-EHgO crys ta l mounted
and oriented on a goniometer head fix e d to a graduated stage and
114
between crossed p o la rize rs indicated p a ra lle l e x tin c tio n in the a-b
plane and an e x tin c tio n angle of approximately 30° from the a_ axis in
the a-c plane.
With respect to the chromophore these e x tin c tio n d i­
rections are id e n tic a l to those found by Ferguson and Wood in the
dihydrate CoClg-ZHgO (3 9 ).
Extremely th in plates of the pure Cu s a lt are e a s ily grown, y ie ld ­
ing samples fo r spectra o f the a-b cross section.
Large c rystals are
also re a d ily obtained which may be s lic e d in the proper o rie n ta tio n to
give access to the remaining d ire c tio n necessary fo r a complete set of
polarized spectra.
Thus, i t is th e o r e tic a lly possible to solve the
problem presented by the monoclinic stru c tu re of the pure Cu s a lt .
However, decomposition o f the pure Cu crystal when exposed to the IR
source made i t impossible to obtain the desired p o la riz a tio n data.
In the Cary 14 the IR source, a high in te n s ity tungsten lamp, is
located in the compartment adjacent to the sample c e ll.
The extreme
s e n s itiv ity o f the Cu TMA crystals to the IR source may be explained by
th is proxim ity, coupled with the fa c t th a t the sample is irra d ia te d by
the e n tire output o f the source throughout the scan.
There are several possible explanations fo r th is decomposition:
one is loss o f water which seems to be a problem fo r th is hydrated s a lt
even a t room temperature.
another explanation:
Previous work done in th is lab (52) o ffe rs
a photochemical redox reaction may take place be­
tween the C u (II) and the chloride ions.
E ith e r or both o f the above
may occur during a scan, rendering the re s u ltin g spectrum unusable.
Mixed metal c rystals containing Cu can be grown in e ith e r the o r­
thorhombic or monoclinic space group;
thus, the stru ctu re problem can
115
be circumvented by selecting a mixed metal crystal with a low mole fra c ­
tio n o f Cu fo r spectral wqrk.
The Mn-Cu crystal with the le a s t Cu was
chosen both because o f it s la t h - lik e habit fo r easy o rie n ta tio n and it s
la c k .o f in te rfe rin g peaks due to the presence of the M n (II) ion.
The
mixed metal crys ta ls proved more durable than the pure Cu s a lt , but
they were also susceptible to damage by the IR source.
Despite a ll the
precautions taken, inform ation on the C u (II) polarized spectra gained
from the Mn-Cu crys ta ls is not as complete as th a t fo r some o f the other
ions studied.
Even though the crystal remained in ta c t during a scan,
the in te n s ity of tra n s itio n s due to the d
ion drove the peaks o f f scale.
As the crys ta ls were ground th in n e r, decomposition once again occurred.
Low temperature spectra would have been h e lp fu l, but th is c a p a b ility
was unavailable on the Cary 14.
This problem with in te n s ity of the
chlorocuprate spectra has been observed by other workers as w e ll.
There is a great deal of lit e r a t u r e data concerning C u (II) spectra,
but few workers have looked a t th is ion in
symmetry (5 3 -5 5 ).
Hitchman and Cassidy have studied two s a lts in which the planar CuCl
u n it is present;
these are (methylphenethylammoniurn),,CuCl4 (54) and
(creatiniuiri^CuCl^ (55) h e re a fte r designated as A2CuCl4 and A 12CuCl4 .
There is no axial lig a tio n in these compounds, the nearest axial atom
O
ly in g more than 3 .0 A d is ta n t from the central Cu atom; there are two
0
O
s lig h tly d iffe r e n t Cu-Cl bond lengths, 2.23 A and 2.27 A in A 12CuCl4
O
O
and 2.25 A and 2.28 A in A2CuCl4 , making the local s ite symmetry fo r
these complexes D2^.
Hitchman and Cassidy have assigned tra n s itio n s
9
fo r the d ion from which the r e la tiv e s p lit t in g of the d o rb ita ls can
be determined.
This is presented in Figure 34;
they noted th a t the
116
dz2
o r b ita l was lowered in energy much more than was expected.
The
energy level diagram based on symmetry representations along with pola­
riz a tio n data is shown in Figure 3.5.
A2CuCl4
TMACuC I3- 2H20
S p littin g Arrangement
S p littin g Arrangement
x2-y 2
----- Z
-
- -
*9 <
xy
t ~ < i x ------- ---2g
Figure 34
x V
xz
yz
yz
XZ
Z2
xy
S p littin g o f the d O rbitals in D2^
Although the o rb ita l s p lit t in g pattern proposed by Hitchman and
Cassidy (54) f i t s the spectra fo r the two planar complexes, i t cannot
explain th a t o f the TMA s a lt .
Only low temperature data were reported
fo r A2CuCl4 , but the room temperature spectra of A 12CuCl4 in one po­
la r iz a tio n is v ir t u a lly id e n tic a l to one o f the polarized spectra of
the TMA s a lt .
However, the p o la riz a tio n e ffe c ts predicted by group
theory do not match even when the c e ll axes are compared w ith respect
to the chromophore o rie n ta tio n .
Therefore, one must assume th a t a
d iffe r e n t s p lit t in g arrangement is responsible fo r the C u (II) spectra
117
Representation
m Oh
Representation
in D2h
/
. /
/
/
/ A
Eg <'x
I
I
I /
T2g
CO
I
/
Forbidden
P o la riza tio n
Calculated
Observed
Peaks
none
588
none
X
692
X
y
712
y
Z
800 .
Z
Forbidden
P o larizatio n
Observed
'
\
\
S
v_Big
\ 1
■N
X
x A
Energy Level Diagram fo r AgCuCl4 in D
2h
Figure 35
Representation
- m Oh
Representation
^n D2h
Forbidden
P o la riza tio n
Calculated
Observed
. Peaks
Forbidden
P o larizatio n
Observed
Z
805
Z
y
860
y
X
895
y?
960
none
PO
IO
I
/
Z
V
I
Co
I
^2g
I
I
I
/ S
X
E
- r "
Figure 36
-3 g
Ag
none
Energy Level Diagram fo r TMACuCl 3 'ZHgO in Dgh
118
in the TMA s a lt .
There is a x ia l lig a tio n in TMACuCly 2H20;
the Cu-O bond length is
about 2.0 A and the two Cu-Cl bond lengths are 2.3 A and 2.8 A;
An­
other d iffe re n c e between the d ihydrate and Hitchman's s a lts is the ex­
te n t o f the d is to rtio n w ith in the CuCl4- plane.
The Cu-Cl distances
O
d i f f e r by only 0.04 A in AgCuCl4 whereas in the TMA s a lt they d if f e r by
O
0 .5 A.
These d ifferences in geometry seem to favor a d iffe r e n t o r b it ­
al s p litt in g pattern in TMACuClg-2HgO.
To compound the problem the
C u (II) ion has been doped in to an orthorhombic s ite dominated by the
,
'
O
M n (II) ion.
In TMAMnClg -2Hg0 the Mn-O bond distance is 2.2 A; the
°
0
q
Mn-Cl bond distances are 2.5 A and 2.6 A;
This suggests th a t the d
ion is forced in to an octahedral s tru c tu re where the a x ia l bond lengths
are shorter than the equatorial bond lengths, although the actual en­
vironment is probably somewhere between th a t of the two pure end mem­
bers, o f the TMA s e ries .
Several possible s p lit t in g arrangements e x is t fo r the d o r b ita ls ;
the one proposed in Figure 34 can be used to construct an energy level
diagram s u ita b le fo r the explanation o f the observed po larized spectra
o f the Mn-Cu TMA s a lt .
Symmetry representations based on th is s p li t ­
tin g arrangement are shown in Figure 36.
There are only fiv e possible electro n c o n fig u ra tio n s .fo r the d®
ion.
Placing nine electrons in the d o rb ita ls resu lts in the same
symmetry as placing one hole in these fiv e o r b ita ls .
Therefore, the
symmetry o f the o rb ita l occupied by the unpaired electron w ill be the
symmetry o f the s ta te .
The d o rb ita l s p lit t in g pattern proposed fo r
the TMA s a lt in Dg^ is reasonable in th a t the Cu-O bond which defines
119
the z_ axis is the shortest bond;
making the z
2
the most repulsion w ill be along z.9
o r b ita l the highest energy le v e l.
w ith in the CuCl^- plane;
The x and ^ axes l i e
the Cu-Cl bonds are longer than the Cu-O
bond, so there is le a s t repulsion along x a n d
Presence of the
C u (II) ion d is to rts the nearly square planar CuCl^" arrangement;
w ill remove the degeneracy o f the xz and yz o r b ita ls .
th is
The tgg t r i p l e t
is s p li t so th a t the xy o rb ita l has the lowest energy, the xz o rb ita l
is s lig h tly higher and the yz o r b ita l is the highest in energy of these
th ree.
I f nine electrons are to occupy the d o rb ita ls as s p li t in Figure
34, the ground s ta te has the symmetry o f the d^2 o r b i t a l, Ag in
The lowest energy excited s ta te is found by promoting an electron from
2 2
2
the x -y . o rb ita l to the z o r b i t a l;
s ta te is also A .
9
the symmetry o f th is excited
Transferral o f an electron from the y z , xz or xy
o rb ita ls y ie ld s excited states with representations B3g, Bgg and Bl g ,
re sp ec tiv e ly .
This gives ris e to the energy le v e l diagram presented
in Figure 36.
Spectra were taken of the b-c cross section o f the Mn-Cu c ry s ta ls ;
in y_-polarized lig h t these c rystals are dark amber, in z-p o la rized
lig h t they are lig h t green.
Four peaks can be id e n tifie d in the near
IR fo r the C u ( II) spectra obtained, but only one remains on scale.
Due to the h a b it o f the Mn-Cu c rystals only the y - and z-p o la rize d d i­
rections were accessible.
A large peak, the top of which is o ff
s c ale, appears in the near IR in ^ -p o la riz e d lig h t ;
dicates th a t the top o f th is peak lie s a t 805 nm.
lig h t a broader double peak appears;
in te rp o la tio n in ­
In ^ -p o la rize d
the tops o f both are o f f scale.
120
but in te rp o la tio n suggests peak maxima a t 860 and 895 nm.
In both of
i
these po larized spectra a 'w e ll-d e fin e d shoulder is evident a t 960 nm.
The peak a t 960 nm is assigned as the Ari
unpolarized.
A„ tra n s itio n which is
The peak a t 895 nm is the only one which does not f i t as
an e le c tr ic dipole tr a n s itio n ;
th is IR peak seems to disappear in %-
po larized lig h t whereas i t should only disappear in x -p o larize d lig h t .
Because the IR region was scanned ra p id ly to preserve the c r y s ta l, th is
peak may be hidden under the large peak seen a t 805 nm.
There is
some s lig h t evidence o f a shoulder in the proper region, i f one exam­
ines the spectrum c lo s e ly .
Ag
Bgg tra n s itio n ;
in y -p o la riz e d lig h t .
The peak a t 860 nm. is assigned as the
i t is v is ib le in ^ -p o la rize d lig h t and vanishes
The highest energy peak in the IR region occurs
a t 805 nm and is assigned as the Ag -*• B^g tra n s itio n which is forbidden
in z-p o la rize d lig h t .
The x-p o larize d spectrum would have provided
valuable addition al d a ta , but could not be obtained due to the habit of
these c ry s ta ls .
In the v is ib le portion o f the Mn-Cu spectrum a peak appears a t
548 nm in ^ -p o la rize d lig h t ;
i t is not present in ^ -p o la riz e d lig h t .
This peak is te n ta tiv e ly assigned as a charge tra n s fe r band occurring
a t r e la tiv e ly low energy;
th is band is presumably the o rig in of the
amber color o f the crystal in th is o rie n ta tio n .
The peaks due to the
M n (II) ion can be seen in the ^ -p o la rize d spectrum, but the in te n s ity
goes o f f scale soon a f t e r the observed peak in the ^ -p o la riz e d spectrum.
Crystals o f (CHgNHgJgMnCl^ and (enHg)MnCl^ when doped with small
amounts o f C u (II) are deep red;
W ille t t has reported a polarized peak
a t 480 nm in these crys ta ls which is re sp o n sib le.fo r the color (5 8 ).
121
The chromophore fo r th is peak is CuCl5 whereas the chromophore fo r the
peak seen in the TMA s a lt is CuCl4 (H2O)2 .
The Cu-Cl bonds are longer
in the l a t t e r , and two chlorides have been replaced by w ater, which
should weaken the crystal f ie ld in te ra c tio n s .
Thus, these two peaks
may have s im ila r o rig in s .
One other system has been reported in the lit e r a t u r e which con­
tains a Mn-Cu m ixture;
have been studied (5 9 ).
dimers of Cu(C5H5NO)Cl2- H2O and Cu(C5H5NO)2C l 2
The doped c rystals also e x h ib it a color
change from the pure compounds (6 0 ).
W i lle t t , e t. al_., (58) worked with a very small range o f mole
fra c tio n s , claiming th a t the C u (II) ion cannot be forced to occupy the
M n (II) s it e , even though the space groups are, id e n tic a l fo r both end
members.
Work done by Campana a t th is campus (61) indicates th a t the
e n tire range is possible fo r (CH3NH3 ) 2MnxC u ^ xCl4 .
There also seems
to be no problem g e ttin g any mole fra c tio n in the TMA series although a
s tru c tu ra l change occurs as Xqu -*■ I .
The existence o f a d iffe r e n t color in mixed metal substances con­
ta in in g Mn and Cu than is present in e ith e r pure end member is well
documented.
The source fo r the peak in the v is ib le region which causes
th is color change has been postulated to be an electron tra n s fe r of
some s o r t, e ith e r -n-^ to Cu or Mn to Cu.
I f an electron tra n s fe r oc
curs between Mn and Cu, th is peak should be seen in a ll Mn-Cu systems;
th is has not been obs
ved.
Therefore, the charge tra n s fe r from Cl to
Cu induced by local s ite in tera c tio n s seems more probable.
122
CONCLUSION
Orthorhombic c rystals of T IW y ij^ xC l 3 * 2H20 where M and M1 repre­
sent the d iv a le n t tra n s itio n metal ions Mn, Fe, Co, Ni and Cu were
found to e x is t as s o lid solutions.
The ease with which these ions are
incorporated in to the la t t ic e s ite s was established:
Ni < Co < Mn < Cu.
S o lu b ility increases as the size of the ion increases;
Cu is unusually
soluble because J a h n -T e lle r d is to rtio n increases the e ffe c tiv e size of
th is aon.
Cell parameters were determined fo r the mixed metal s a lts
and also fo r the previously unpublished pure Fe and Ni analogs.
These
values f ji t the trend expected, i . e . longer c e ll dimensions fo r the
la rg e r cations.
Because these highly colored crys ta ls are orthorhombic, th e ir u n it
c e ll axes are e a s ily oriented with respect to the e le c tr ic vecto r, t , of
polarized lig h t .
U t iliz in g D2^ symmetry to account fo r the pleochro-
ism exhibited by these c ry s ta ls , energy level diagrams were constructed
to f i t the observed po larized spectra.
In the M n (II) spectra one peak
appears which must be assigned as a magnetic dipole tra n s itio n ;
remainder were assigned as e le c tr ic dipole tra n s itio n s .
the
In the C o (II)
spectra a ll o f the observed peaks must be assigned as magnetic dipole
tra n s itio n s .
One peak remained unexplained as e ith e r a magnetic or an
e le c tr ic dipole tra n s itio n ;
th is "anomalous" band needs fu rth e r study
before it s o rig in can be f u l ly understood.
In the F e ( II ) and N i ( I I )
spectra in s u ffic ie n t p o la riz a tio n data was obtained to estab lish and
adequately support the r e la tiv e s p lit t in g pattern in a
diagram.
energy level
In the C u (II) spectra the peaks were assigned s o le ly as
e le c tr ic dipole tra n s itio n s with a charge tra n s fe r band appearing in
123
the v is ib le region.
In a ll cases the observed spectral data corre­
la te well w ith those in the lit e r a t u r e fo r ions in s im ila r environ­
ments.
Crystals with lower C u (II) concentrations and higher N i ( I I ) con­
centrations are necessary to v e rify the exact s p littin g pattern in
symmetry fo r these ions.
Low temperature spectra o f these crystals
would provide a means to distinguish between magnetic and e le c tr ic d i­
pole tra n s itio n s experim entally ra th e r than so le ly by group th e o retical
pred ictio n s.
124
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MONTANA STATE UNIVERSITY LIBRARIES
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An interpretation o f the polarized cryst
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L in d b e c k , M a r th a J e a n
A n in te r p r e ta tio n o f
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DATE
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