Nephelauxetic effect for the isoelectronic 3d3-ions (Cr3+, Mn4+, Fe5+) in SrTiO3

The exchange charge model of crystal field theory has been used to analyze systematically the ground state absorption spectra of isoelectronic Cr3+, Mn4+, and Fe5+ ions in an octahedral coordination in the SrTiO3 crystal. The parameters of the crystal field acting on the valence electrons of impurity ions are calculated from the available crystal structure data. A special attention is paid to the analysis of dependencies of the crystal field invariants and covalence effects on the impurity ion. It is shown numerically that the covalence effects between the above impurity ions and ligands increase with an increase of the 3d-ion oxidation state.