Hc-QS-IS Correlations in Octahedral Iron Compounds

Abstract

While Mossbauer studies of particular iron compounds have been very useful in illucidating theoretical questions, by determining the symmetry or the character of the electronic ground state, its contribution to the overall understanding of chemical bonding in iron compounds has been rather limited. Present theories of bonding in the first transition metal group have been developed mainly with the results of optical spectroscopy in mind, thus predicting energies of the electronic configurations. Mossbauer data, on the other hand, reflect the charge density distribution (isomer shift, IS, and quadrupole splitting, QS) or the details of the unpaired spin density (magnetic hyperfine interaction). Energy calculations are quite insensitive to significant modifications of the electronic charge density distribution in solids and it is not surprising that the results of such calculations do not contribute much to the interpretation of the Mossbauer data. Furthermore, bonding theories have been developed for the cases of extremely ionic and extremely co- valent compounds and no single theoretical model is available to cover the entire range of covalency and to be used for the interpretation of the experimental results. The situation appears to be such that theories which have been developed to account for the energetics of electronic levels have more or less exhausted the information available from optical spectroscopy and are awaiting for a different type of experimental information for further confirmation and refinement or possible rejection. Apart from energy calculations, good theoretical predictions are available for the free-ion unpaired spin density distribution, as reflected by the magnetic hyperfine interaction at the nucleous [l]. Some attempts have been published to allow for covalency effects, as reflected in the neutron magnetic form factor data [2]. A phenomenological analysis of the available experimental results for octahedral divalent iron compounds yields an empirical model which provides some guidance for futher theoretical investigation [3]. This work will be summarized here and extended to include low-spin iron compounds