Electronic structures of Co(II) and Co(III) impurities in cubic perovskite hosts

The spin-polarized option of the multiple-scattering $X\alpha$ method is used to obtain the electronic structures of divalent and trivalent cobalt ions in SrTi${\mathrm{O}}_3$ and BaTi${\mathrm{O}}_3$ samples. Various clusters account for the different charge-compensation mechanisms (oxygen vacancies, fluorine ions) and different spin configurations are considered for Co(II) and Co(III) centers. Transition-state computations are carried out to get the various multiplet levels of Co(II) and CO(III) cubic centers. The high-spin ${}_{}{}^5{}_{}{}^{}T_{2g}^{}{}_{}{}^{}$ and the low-spin ${}_{}{}^1{}_{}{}^{}A_{1g}^{}{}_{}{}^{}$ terms are, respectively, the Co(III) ground terms in BaTi${\mathrm{O}}_3$ and SrTi${\mathrm{O}}_3$ hosts. The 10Dq crystal-field parameter of Co(III) increases from 1.9 eV (BaTi${\mathrm{O}}_3$) to 2.1 eV (SrTi${\mathrm{O}}_3$). In the light of $X\alpha$ energy diagrams, the possible interpretations of the absorption spectra relevant to Reimeika BaTi${\mathrm{O}}_3$: Co and SrTi${\mathrm{O}}_3$: Co crystals are reviewed.