A simple ligand-field model is shown to yield a unified explanation of numerous low-temperature experiments on the ferrous ion in (Fe(H 2 O) 6 )(NH 4 ) 2 (SO 4 ) 2 . The experimental data are taken from single-crystal Mossbauer spectra in fields from 0 to 5 T, far-infrared Fourier transform spectra, far-infrared EPR spectra and from measurements of the anomalous specific heat, inelastic neutron scattering and the magnetic anisotropy. Reliable information on the low-lying electronic states of the ferrous ion including its wavefunctions and hyperfine interaction is obtained. The electron density distribution of the ground state has the shape of a cigar which points almost exactly to the centre of a triangle formed by the oxygen ligands. The local symmetry at the iron site was found to be only C i . It turns out that some methods used frequently to investigate the electronic properties of the ferrous ion may lead to results whose ambiguities are not recognised. Among these are the analysis of the temperature dependence of the nuclear quadrupole splitting from Mossbauer spectra and the analysis of Mossbauer spectra for powdered samples in magnetic fields.
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R. Doerfler (1987) studied this question.