CRYSTAL-FIELD EFFECTS ON HIGH-SPIN FERROUS ION

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Abstract
To treat the electronic Hamiltonian of the ferrous ion in a crystalline field, when only the high spin 5D term is concerned, several methods may be used. We describe the most general one, and indicate some effects of the crystal-field upon the spontaneous or induced hyperfine interactions. We review recent work aiming to determine crystal-field level schemes, and compare the values given for the relevant parameters : 1, < r-3 > 3a, AS, AEo. We also describe a few systems for which the basic assumptions - time and temperature independent crystal field, fas relaxation - are not fulfilled. 1. Introduction. - The physical basis of the crystal- field model has been described by several authors in books mostly devoted to the electron paramagnetic resonance and to -the magnetic properties of the transition ions (I). We do not describe it again, and restrict ourselves to the Mossbauer studies of the fer- rous ion in the 5D manifold. The Mossbauer effect usually provides information on the electronic level scheme of Fe2+ in the crystal- field approach. In some cases low-temperature mea- surements allows the electronic ground-state to be determined (2-51. By using both low-temperature Mossbauer data and direct measurements, such as EPR, IR, Raman, optical spectroscopies, the level scheme can be completely determined : an early example is that of Fe2+ in ZnF, and FeF, 141. Magne- tic data also are useful for this determination. When only Mossbauer data are available, additional information is needed and may come from the thermal dependence of the hyperfine interactions (6). The assumption of a temperature independent crystal field is then required. Following previous works (6-81, this theory has been applied to many ferrous compounds, often successfully. In the present study, we review recent work dealing with crystal field effects on the high-spin ferrous ion. We also describe the general calculation in the 5D manifold, and outline the various difficulties occuring in the development of crystal-field studies.

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