Abstract
The fourth-order perturbation formulae for the zero-field splitting parameters D and E of a 5D ion in D2 symmetry are derived by taking the spin-orbit and the spin-spin interactions as the perturbation Hamiltonians. From the EPR and optical spectra data of Fe II in FeF2 it has been found that the orbital ground state for FeF2 is in the form a mod x2-y2)+b mod 3z2-r2), rather than mod yz) as suggested by some authors. This conclusion is consistent with those of Abragam and Boutron (1961) and Christoe and Drickamer (1970) who determined the ground state from the analysis of Mossbauer and optical spectra data.