Bull. Mater. Sci., Vol. 19, No. 4, August 1996, pp. 607-613. ,c, Printed in India.
Synthesis, structure and IR absorption studies of L n B a C u C o O s
(Ln = rare earth) oxides ~
C S H I V A K U M A R A , M S H E G D E *t and G N S U B B A N N A ÷
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012,
India
÷Materials Research Centre, Indian Institute of Science, Bangalore 560012, India
MS received 1 March 1996
Abstract. A series of oxides LnBaCuCoO5 (Ln = Pr, Nd, Sin, Dy, Gd, Ho and Er) have been
synthesized by ceramic method. The oxides crystallize in a tetragonal structure, isostructural
to YBaCuCoO 5. All the oxides in the series are semiconducting. IR spectra of these oxides
show distinct absorption bands at 630 cm 1,550 cm- 1and 330 cm - ~which are assigned to E,
A2 and A 1 modes respectively. Doping of holes in these oxides, by calcium substitution in
Erl- xCa~BaCuCoO~ x (up to x ~ 0.31 was done but, these oxides did not show metallic
behaviour.
Keywords. Ceramic method; rare earth oxides; IR spectra.
1.
Introduction
Since the discovery of a new defect l a y e r e d perovskite, Y B a C u M O 5 (M = F e a n d Co)
( E r - r a k h o et a11988; B a r b e y et a11992), there has been a c o n s i d e r a b l e interest in these
classes of m a t e r i a l s to u n d e r s t a n d their crystal structure a n d m a g n e t i c p r o p e r t i e s
( H u a n g et al 1994; M o m b r u et al 1994; C a i g n a e r t et al 1995). Spin state a n d spin
o r i e n t a t i o n of C o ( I l l ) ( B a r b e y et al 1994) a n d M 6 s s b a u e r studies in L n B a C u F e O 5
(Ln = rare e a r t h ion except La) have been investigated (Pissas et a11992). F u r t h e r , solid
s o l u t i o n s of Y B a C u x C o 2_xO5 (0 < x < 1) ( W u u z o n g 1994) have been studied a n d in
these oxides an i r r e g u l a r i n t e r g r o w t h of Y B a C u C o O 5 with Y B a 2 C u 3 0 7 has been
observed. In this respect, the l a y e r e d 112-type oxides are interesting materials, which
are built up of c o r n e r s h a r e d C u O 5, a n d ( F e / C o ) O 5 p y r a m i d s . Its structure can be
r e g a r d e d as a derivative of the h i g h - T c s u p e r c o n d u c t i n g YBa 2 C u 3 0 7 phase, achieved
by r e m o v i n g one B a - O layer a n d C u - O chain with 50% of the r e m a i n i n g C u a t o m s
being substituted by C o o r Fe. W e c o n s i d e r e d w o r t h w h i l e to synthesize L n B a C u C o O 5
(Ln = rare e a r t h ion) series of oxides a n d s t u d y their s t r u c t u r e a n d properties. F u r t h e r ,
s u b s t i t u t i o n of d i v a l e n t a l k a l i n e e a r t h ion for the l a n t h a n i d e ion m a y lead to hole
d o p i n g giving a metallic phase. Here we r e p o r t synthesis, s t r u c t u r e a n d p r o p e r t i e s of
L n B a C u C o O 5 oxides.
2.
Experimental
T h e s a m p l e s were p r e p a r e d b y a ceramic m e t h o d . T h e p a r e n t oxides Ln 2 0 a (99-6%)
( P r 6 O l a in case of Pr), CUO(99%), C o 3 0 4 (99%) a n d B a O 2 (99"5%) were mixed
stoichiometrically, g r o u n d well a n d h e a t e d up to 9 3 0 - 1 0 0 0 ° C with one i n t e r m i t t e n t
tContribution no. 1175 from Solid State and Structural Chemistry Unit.
607
608
C Shivakumara, M S He#de and G N Subbanna
Table I. Synthetic conditions and lattice parameters of LnBaCuCoO~
(Ln = rare earth ion).
Lattice parameters (~)
Composition
Synthetic conditions
950'C /48h
95ffc /48h
930'C 48h
930"C /48h
930"C /48h
930"C /48h
930"C /48h
IO00"C /48h
PrBaCuCoO 5
NdBaCuCoO s
SmBaCuCoO~
GdBaCuCoO5
DyBaCuCoOs
HoBaCuCoO 5
ErBaCuCoO.~
YBaCuCoO~
.k00
_Gd-------k___
3.926( I )
3.916(7)
3.874(9)
3-875(2)
3.873(4)
3.872(4)
3-871 (7)
3.867(3)
7'668(7)
7-658(3)
7.597(5)
7.587(8)
7.548(4)
7.538(1)
7-523(8)
7.571(5)
.... J
c
E
8A
" ' - . . . . . i-
10
Figure I.
2O
8
I
30
40
20 (Cu Ko()
I
50
Powder X-ray diffraction pattern of LnBaCuCoO 5.
60
Synthesis, structure and IR absorption studies of LnBaCuCoO 5
609
grinding, and then furnace cooled. The synthetic conditions are given in table 1.
Powder X-ray diffraction of these samples were recorded in a JEOL JDX-8P diffractometer. Oxygen estimation was carried out by thermogravimetric analysis (TGA) in
hydrogen atmosphere. The IR spectra of these samples were recorded in a Perkin Elmer
spectrometer using pressed KBr pellets. The electrical resistivity measurements were
done using a four-probe method. Transmission electron microscopy of selected
samples were carried out to confirm the structure.
3.
Results and discussion
The powder X-ray diffraction patterns of LnBaCuCoO 5 (Ln = rare earth ions) are given
in figure 1. For comparison the XRD pattern of YBaCuCoO 5 is also given. The X-ray
pattern could be indexed on a tetragonal cell with P4/mmm space group and the cell
parameters of all the samples were obtained by least-square fitting, as summarized in
table 1. These samples were synthesized at relatively lower temperatures as compared to
YBaCuCoO 5. The variation of lattice parameters, a and c as a function of rare earth ionic
radii is given in figure 2. We clearly see from this figure that both a and c are increasing
with increase in the rare earth ionic radii. The indexed powder X-ray diffraction data of
a typical member, ErBaCuCoO5 is given in table 2. The observed intensity matched well
with the calculated values obtained by Lazy Pulverix programme. Thus, from figure 1
and tables 1 and 2, LnBaCuCoO 5 (Ln = Pr, Nd, Sm, Dy, Gd, Ho and Er) are shown to
form single phase oxides which are isostructural to YBaCuCoO 5 phase. However, the
lanthanum analogue of this phase could not be synthesized. When stoichiometric
mixtures of La203 and the oxides were heated at 950-1000°C, a cubic like phase was
obtained with a ~ 3.94 ~, probably due to mixing of La ÷ 3 and Ba ÷ 2 sites.
The TGA curve of a typical member, ErBaCuCoO 5 is given in figure 3. Oxygen
estimated from the weight loss corresponds to the formula ErBaCuCoOts.0o6_+o.oo3).
3.96
7.90
- 7.80
"~3.92
U
7.70 N
E
E
a.88
0
Cl.
0 0
0
0
7.60 a.
U
._
3.84
3.80
0.95
7,50 q
i
1.00
l
I
l
1.05
1.10
1.15
Ionic radii (,~)
1.20
Figure 2. The lattice p a r a m e t e r s a and c for L n B a C u C o O 5 as a function of the rare e a r t h
ionic radii.
610
C Shivakumara, M S Hegde and G N Subbanna
Table 2. Powder
Er BaCuCoO s
X-ray
diffraction
data
for
h
k
I
dob~(/~),
d ,(/~)
lob~
I
0
!
0
1
1
1
0
1
1
2
0
2
1
2
1
0
0
0
0
1
0
0
1
0
0
0
0
0
1
1
1
0
2
1
0
2
3
2
3
0
4
2
4
2
4
7"531
3-867
3"769
3"440
2"726
2'702
2.512
2'214
2"106
1"928
1"882
1"726
1"691
1'572
1"548
7"523
3-871
3"762
4"442
2.737
2"698
2-509
2"213
2"105
1"936
1-881
1-721
1-692
1"573
1"550
3
19
10
< 1
46
100
4
19
3
23
7
3
5
25
14
4
11
7
< 1
45
100
3
20
3
30
9
4
5
37
19
D
0.129 ~,,-..--. •
0.127
0.125
E
,.~ 0.123
0.121
0.119
0.11'~
20
L
i
i
[
220
I
I
I
I
,
,
420
,
I
620
,
,
i
l
,
910
Ternp ("c)
Figure 3.
Thermogravimetric curve for ErBaCuCoO r.
Even after annealing these samples in oxygen atmosphere there were no significant
changes in the oxygen content. From these results we believe that our samples are
nearly stoichiometric LnBaCuCoO 5.
Transmission electron microscopic studies were carried out on a typical member,
ErBaCuCoO 5, to confirm the structure. Figures 4a and b show lattice image and the
corresponding diffraction"pattern. The lattice image with 7'5,~ fringe spacing confirmed the layered nature. The electron diffraction pattern recorded along (00 1)
direction indicated the tetragonal cell with lattice parameter a ~- b 2 3'9/~.
The IR spectra of LnBaCuCoO 5 samples are shown in figure 5. We can clearly see
that, as the rare earth ionic size increased, intensity of absorption bands decreased. The
decrease is prominent in the case of Pr and Nd. In all these oxides, three distinct
absorption bands were observed at 630 cm- 1 550 cm- 1 and 330 cm- 1. For comparison, the IR spectra of La 2 CoO 4 are given in figure 5, which agrees well with the spectra
Synthesis, structure trod I R absorption studies ~?/ L n B a C u C o O 5
611
Figure 4. a. Latttce image of ErBaCuCoO s with fringe spacing 7-5 ~ and b. selected area
diffraction pattern of image shown in a.
reported in the literature (Sreedhar and G a n g u l y 1990). In L a 2 C o O 4, C o is octahedrally coordinated to oxygen. The absorption bands of L a 2 C o O 4 at 6 3 0 c m -1,
5 1 0 c m - t , 4 1 0 c m - 1 and 330cm I are assigned to in-plane stretch E u, out of plane
A 2 and two A ~ modes respectively. In the case of L n B a C u C o O 5 system, due to the loss
612
C Shivakumara, M S Hegde and G N Subbanna
.fi
Lo
tn
c
o
I--
800
600
400
200
cm -I
Figure 5. Infrared absorption spectra of LnBaCuCoO 5.
11
10
O,,C
..J
7
3.2
t
I
3.6
t
I
i
4.0
IO00/T.K
1
-I
4,4
i
I
4.8
Figure 6. Logarithm of resistivity as a function of reciprocal temperature for compounds
(u) DyBaCuCoO 5 and (b) HoBaCuCoO~.
Synthesis, structure and IR absorption studies of LnBaCuCoO s
613
of octahedral s y m m e t r y one of the vibrational modes (A 1 ~ along z-direction should
disappear. Accordingly, we saw the absorption bands at 6 3 0 c m - ~ (E), 5 5 0 c m - ~ (A 2)
and 330 cm - ~(A t ). One absorption band near 410 cm - ~ disappeared. The I R spectrum
of Y B a C u C o O 5 was similar to L n B a C u C o O 5 oxides, indicating structural similarity
a m o n g s t these oxides.
The electrical resistivity measurements were done by the four-probe m e t h o d using
sintered pellets. All these oxides are semiconducting. Typical ln(pl vs 1000/T plots for
Ho and Dy are given in figure 6. The band gaps Eg of these oxides were in the range of
0'09 eV.
In an attempt to dope holes in these layered oxides, we synthesized the Er~ xCax
BaCuCoO~, (0 < x < 0.3) series of oxides at 920 C for 48 h and furnace cooled. The
compositions for x > 0.3 were multiphasic. The powder X R D pattern could be indexed
on a tetragonal cell, like the parent phase. However, there was no significant change in
the resistivity of these doped c o m p o u n d s and they are semiconducting.
4.
Conclusions
We have synthesized a series of LnBaCuCoO.~ (Ln = rare earth ion) oxides related to
Y B a C u C o O 5 type of phase. The unique feature of these class of materials is the
presence of ( C u / C o ) O 5 corner shared pyramidal layers, which showed distinct IR
absorption s p e c t r u m An attempt to induce metallicity in this phase by hole doping was
not successful.
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