Zero and Applied Field Mössbauer Spectroscopy
Investigations of Magnetic Frustration in Certain
Classic Werner Complexes Above and Below 1K.
aWilliam
M. Reiff, a,bLázlo Tákács, a,cMichael J. Kwiecien
aDepartment
of Chemistry, Northeastern University , Boston, MA 02115 ,USA,
w.reiff @ neu.edu, bDepartment of Physics, University of Maryland, Baltimore
County, Baltimore, MD, USA and cGillette Corporate Research, Boston, MA 02115,
USA
[Ru(NH3)6][FeCl6](I) is an (S=1/2, S =5/2) system whose structure is found to be monoclinic (C2/c at
293K) and isomorphous to the (S=3/2, S=5/2) [Cr(NH3)6][FeCl6] (II). The lower symmetry of (I) and
(II) relative to the cubic (Pa3 at 293K) [Co(NH3)6][FeCl6] (III) (S=0, S=5/2) is reflected in a resolved
electric field gradient (quadrupole splitting effect) at the complex FeCl63-anion sites. These systems
can be regarded as composed of interpenetrating fcc (Z=4) cation and anion sublattices, (I) and (II)
approximately, (III) more rigorously ( see slides 3 and 4). Assuming that nearest neighbor AntiFerromagnetic exchange interactions are stronger than the next nearest neighbor interactions, one
expects a classic (two sublattice) collinear Néel ferrimagnet as the most likely long range ordered
3D magnetic ground state. However, our Mössbauer Effect characterization of (I), II and III indicate
the possibility of a more complex, at least four sublattice ground state magnetic structure (capable
of approach to “zero” net moment at low T) and in which next nearest neighbor magnetic
interactions are important and perhaps even stronger than the nearest neighbor exchange. This
reflects avoidance of the magnetic frustration that is known to arise for (among other geometries)
the A F interaction of nearest neighbors in a fcc structure. A somewhat less complex example of
geometric magnetic frustration is given for the 2D triangular lattice (slide 4). Here the spins have xy
symmetry and lie in the plane of the triangle. The spins can not simultaneously interact to minimize
the energy of all of the moments for the A F ground state. With destabilization of the standard Néel
A F ground state, degeneracy results as a compromise to minimize the energy of the components
of system. This leads to the curious outcome that familiar long range 3D order may not be
possible.
Greedan, J. E., J. Mater. Chem., 2001, 11, 37-53.
Zhang, J. H.; Reiff, W. M.; Helms, J. H.; Hatfield, W. E., Inorg. Chem., 25, (1986) 2936.
Helms, J. H. ; Hatfield, W. E.; Kwiecien, M. J. and Reiff, W. M.; J. Chem. Phys., 84(1986) 3993.
Hatfield, W. E.; Reiff, W. M.; Takacs, L.; Ensling, J. ; Transition Met. Chem.17(1992) 204-208.
In what follows, we look at this situation from the vantage point of zero and applied field Mössbauer
spectroscopy and complimentary magnetization and crystallography studies.
Cubic or quasi cubic structure of certain [M(NH3)63+] [M’Cl63-]complexes based
on interpenetrating fcc lattices
[Co(III)(NH3)63+] [Fe(III)Cl63- ]
Fcc lattices interpenetrating to the
extent of one half the body diagonal*
yielding the …………
……Halite (NaCl) rock salt structure in
which each mono atomic or complex
ion is six coordinate.
The geometrically frustrated equilateral triangular spin plaquette where all three spins can not
simultaneously minimize their energy via A F exchange thus leading to frustration and ground
state degeneracy.
A recyclable carbon sorption pumped He-3 cryostat convenient for T~ 0.30K to ~6K, a
necessity for spectroscopic investigation of the critical behavior of many of the systems
of this vignette.
Excellent source to detector
geomerty, ~3.5” horizontal
The temperature evolution of magnetic hyperfine splitting confirms long range magnetic
order but not the nature (anti-ferro-, ferro-, ferri-etc.) of the ground state. This can be
ascertained via magnetization (top slide 6) or applied field Mössbauer spectra (slides 8,9).
The dependence of eff
vs T confirms A F
exchange (curve C)
rather than zero field
splitting, curve D. The
value of eff in the limit
of large axial zero field
splitting, D, is 191/2
(4.36) vs ~ 3.15 
observed.
The spectra of slide 5 in
combination with the
data of slide 7, bottom
right confirm 3d AF
order with TNéel of
~0.85K for [Co(NH3)63+]
[FeCl63-], an (S=0,
S=5/2) system. Spectra
for the Cr3+ analogue are
shown below with
TN~2.8K, slide 7 bottom
right.
[Cr(NH3)63+ ] [FeCl63-]
S=3/2, S= 5/2
Calculation of Hinternal for the Cr3+ compound
Transition pairs (2&4)
and (3&5) of the figure on
the right are seen to have
the same excited state
terminus, namely (3/2, 1/2) and (3/2, +1/2)
respectively. Hence the
internal field corresponding
to the I = 1/2 (ground
state) Zeeman
spitting…….
Quadrupole Splitting = 0
TN
……is given by the
average of the quotients
of 2-4 and 3-5 in mm/s
by the factor 0.1188
mm/s/Tesla where the
latter factor arises from
and is specific to the
nuclear moment and g
factor for I=1/2 ground
spin state of Fe57.
Quadrupole Splitting  0
TN
The H vs T phase diagram of an easy axis uniaxial antiferromagnet for
which H(Anisotropy) is less than H(Exchange). The A F to Spin Flop
transition is first order while the Spin Flop to Paramagnetic is second order.
In the context of
applied field
Mössbauer
spectroscopy, H0 is
applied parallel or
perpendicular to E
while  in the table
below is the angle
between E AND H
internal
Spectra (stick
diagrams below) are
typical of a low
anisotropy A F
system. Relatively
“narrow line width”
transitions observed
(single crystals)
broadened for
powders.
2ND ORDER
1ST ORDER
Magnetic Ground State Determination Via Applied Field Spectra
Intensity
AF
*
*
*
*
*
*
1ST
*
*
*
SF
2ND
*
*
*
P
MI= 0*
Longitudinal and transverse applied field Mössbauer spectra of [Cr(NH3)6][FeCl6]
(S=3/2, S=5/2) (powder) at 1.95K and 0.52K, i.e.at T< TN, respectively, confirming spinflop behavior (in accord with slide 8, bottom) and the antiferromagnetic nature of the
FeCl63- sublattice (unto itself!) as opposed to ferromagnetic necessary for overall
ferrimagnetism of this material.
(MI = 0)
Some Magnetic Texture in Zero Field
Sample Spin-Flopped
Incipient SF to Paramagnetic
Spin-Flopped
Note that as expected, the intensity of the MI=0 transitions increases for H(║) , 1.95K,
and decreases for H() at 0.52K. The antiferromagnetic interaction of the FeCl63-anions is
consistent with geometric frustration for this sublattice.
A F Ordering of [Co(III)(1,2-propanediamine)3 3+][Fe(III)Cl63-](Co(pn)3FeCl6) (S=0,
S=5/2) as confirmed via a combination of susceptibility and zero field Mössbauer
spectroscopy experiments.
TN~9.9K
The effective magnetic moment varies
from ~5.3 at 70K to ~1.5 at 1.6K
Velocity (mm/s) Relative to Fe
In part, we base our interpretation of the A F magnetic exchange pathways in [Co(III)(pn)3]
[Fe(III)Cl6] and the other complexes of this vignette on those shown below for the [Co(NH3)6]
[SbCl6] structure, i.e short N…Cl hydrogen bonds and close interanionic [Sb(Fe)Cl63-] Cl-Cl
contacts. These apparently lead to a remarkably high TN, ~12 times TN of the frustrated
Co(NH3)6 FeCl6, i.e 9.9K vs 0.8K!
Close interionic Cl-Cl
contacts (3.767Å)
Spin-Flop
Transition
N-Cl
Hydrogen
Bonds
(3.545Å)
Or Fe !!
Schroeder, D. R. et al, Inorg. Chem, 12,(1973), 210.
M vs H studies of Co(pn)3FeCl6 up to 1.4T at 1.8K show no field induced transitions. The figure
above indicates complete spin-flopping by 3T while the spectrum is unchanged at 6T. Hence,
1.4T <HSF<3T, a reasonable range for this type of system.
Scoville, A. N; Lazar, K. L.; Reiff, W. M. and Landee, C.,Inorg. Chem. 22,(1983) 3514.
Conclusion
The well known charge transfer behavior (Cl- to Fe3+) is concomitant with metal electron spin
delocalization to the halogen for transition metals (Fe3+, Ir4+) in their higher oxidation states.
With the Mössbauer Effect, we are able to look directly at the cooperative long range
magnetic interaction of these centers of delocalized metal electron spin density as spread
over the halogen atoms of the periphery of the complex anions. For the Co(III) complexes of
this vignette, S = 0 for the complex cation. Hence, the totality of the magnetism of these
complexes must reside in the FCC Fe(III)Cl63- sublattice. Since this sublattice is
geometrically (topologically) frustrated**, the implication is that the A F order of this lattice
results from surprisingly strong exchange interactions (certainly in aggregate) of the
Fe(III)Cl63- anions with very distant next nearest neighbor S=5/2 hexachloro-ferrate(III)
anions of the anion sublattice. The interpretation of the magnetism of [Cr(NH3)6][FeCl6]
presents decidedly more complexity in that this system offers the possibility of two
interpenetrating geometrically frustrated sublattices. Unfortunately, the space available in
this vignette does not allow for further detailed consideration of this system at this time.
**Moron, M. C; Palacio. F; Carlin, R. L.; , Inorg. Chem., 29, (1990) 842.
The zero and applied field Mössbauer spectra of these materials incontrovertibly confirm the
A F ordering of the anion sublattice and are fully consistent with classic magnetization
studies. Even more definitive understanding of the magnetism of these interesting classic
Werner complexes awaits single crystal Mössbauer and polarized neutron diffraction study of
the deuterated analogues.