An assigment of the optical absorption spectrum of mixed valence Co3O4 spinel films

The absorption spectra recorded for Co3O4 and for lightly Ni-doped Co3O4 films both have the regular spinel structure. In the unit cell Co2+ and Co3+ ions share oxygen anion ligands and charge-transfer processes are facile. An assignment of the spectra therefore requires an analysis of the full unit cell rather than treatment of Co2+ tetrahedral and Co3+ octahedral species as individual, non-interacting sites. A semi-empirical form of molecular orbital has been used to construct the energy levels of the chromophore of this unit cell. The charge transfer processes between Co2+ and Co3+ and from oxide ligands to the metal ions have been identified and assigned to the major features of the spectrum.