Covalency Effects on the3d-Charge-Density Distribution in Solid Ferrous Compounds

Abstract

A phenomenological description of the $3d$-charge-density distribution in solids, based on Mössbauer hyperfine interaction data, studies of crystal structures, EPR, neutron scattering and optical spectroscopy, has been developed for divalent iron ions in solids. From this work, it appears that one can obtain an internally consistent picture of the $3d$-charge-density distribution in solids by allowing for large, but distinctly different, modifications of the radial $3d\left(e_g\right)$ and $3d\left(t_{2g}\right)$ electronic wave functions. These modifications vary continuously with covalency on going from the completely ionic to the completely covalent ferrous compounds. They account for the large changes (in order of magnitude) in the mean $3d$ charge (or spin) densities, indicated by the experimental data. These results cannot be explained by theoretical models of bonding based on crystal field and molecular-orbital theories, using free-ion wave functions. The present model emphasizes the importance of the radial modifications of the $3d$ wave functions, thus supporting the point of view that, in self-consistent-field-type theoretical computations, the radial wave functions should be described by variational rather than fixed parameters.