Spectroscopic, magnetochemical, and crystallographic data are presented for CsFe(H
2O)
6PO
4, a member of alittle-kno
wn 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 sho
wn by the first example of an iron(II) Mössbauerspectrum that exhibits no measurable quadrupole splitting. T
wo crystallographically distinct [Fe(OH
2)
6]
2+ complexesare identified from inelastic neutron-scattering (INS) experiments conducted bet
ween 2 and 15 K. The data aremodeled
with the ligand-field Hamiltonian,
![](/images/entities/Hcirc.gif)
=
![](/images/gifchars/lambda.gif)
![](/images/entities/Lcirc.gif)
![](/images/entities/Scirc.gif)
+
B(
k![](/images/entities/Lcirc.gif)
+ 2
![](/images/entities/Scirc.gif)
) +
tet{
z2 - (1/3)
L(
L + 1)} +
rhom{
x2 -
y2},operating in the ground-term
5T
2g (
Oh) basis. An excellent reproduction of INS, Mössbauer, HF-EPR, andmagnetochemical data are obtained in the 2 and 15 K temperature regimes
with the follo
wing parameters:
=-80 cm
-1;
k = 0.8; site A
tet = 183 cm
-1,
rhom= 19 cm
-1; site B
tet = 181 cm
-1,
rhom= 12 cm
-1. Thecorresponding zero-field-splitting (ZFS) parameters of the conventional
S = 2 spin Hamiltonian are as follo
ws: siteA
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 thevariation of the energies of the lo
w-lying states
with respect to displacements along selected normal coordinatesof the [Fe(OH
2)
6]
2+, sho
ws the zero-field splitting to be extremely sensitive to small structural perturbations of thecomplex. The expressions derived are discussed in the context of spin-Hamiltonian parameters reported for the[Fe(OH
2)
6]
2+ cation in different crystalline environments.