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Abstract:
Spectroscopic, magnetochemical, and crystallographic data are presented
for CsFe(H2O)(6)PO4, a member of a little-known isomorphous series of
salts that facilitates the study of hexa-aqua ions in a quasicubic
environment. Above 120 K, the deviations from cubic symmetry are
minimal, as shown by the first example of an iron(II) Mossbauer
spectrum that exhibits no measurable quadrupole splitting. Two
crystallographically distinct [Fe(OH2)(6)](2+) complexes are identified
from inelastic neutron-scattering (INS) experiments conducted between 2
and 15 K. The data are modeled with the ligand-field Hamiltonian, (H)
over cap = lambda(L) over cap(S) over cap + beta B(k (L) over cap +2
(S) over cap)+Delta(tet){(L) over cap (2)(z)-(1/3)L(L+1)} + Delta(rhom)
{(L) over cap (2)(x)-(L) over cap (2)(y)}, operating in the ground-term
T-5(2g) (O-h) basis. An excellent reproduction of INS, Mossbauer,
HF-EPR, and magnetochemical data are obtained in the 2 and 15 K
temperature regimes with the following parameters: I) - 80 cm(-1); k =
0.8; site A Delta(tet) = 183 cm(-1), Delta(rhom) = 19 cm(-1); site B
Delta(tet) = 181 cm(-1), Delta(rhom) = 12 cm(-1). The corresponding
zero-field-splitting (ZFS) parameters of the conventional S = 2 spin
Hamiltonian are as follows: site A D = 12.02 cm(-1), E = 2.123 cm(-1);
site B D = 12.15 cm(-1), E = 1.37 cm(-1). A theoretical analysis of the
variation of the energies of the low-lying states with respect to
displacements along selected normal coordinates of the
[Fe(OH2)(6)](2+), shows the zero-field splitting to be extremely
sensitive to small structural perturbations of the complex. The
expressions derived are discussed in the context of spin-Hamiltonian
parameters reported for the [Fe(OH2)(6)](2+) cation in different
crystalline environments.
