Optical absorption, electrical conductivity and electronic structure in BaTiO₃: Co doped crystals

Abstract

Absorption and electrical conductivity measurements have been carried out on Remeika BaTiO₃: Co doped crystals. The optical spectra shows an absorption band peaking around 2.4 eV which is independent from the impurity concentration. In order to localize the impurity levels inside the BaTiO₃ band gap and to provide a theoretical interpretation of optical and electrical measurements, energy level schemes have been obtained from spin unrestricted Xα computations performed on different clusters. The chosen clusters, for instance CoO₆ ^10-, CoO₆ ^9- are representative of substitutional Co²⁺ and Co³⁺ impurities at the Ti⁴⁺ site without local charge compensation, while the CoO₅Vo and CoO₅F ones, take into account a charge compensation mechanism by oxygen vacancies Vo or fluor ions F. The Xα eigenvalues show that both empty and occupied Co levels are localized inside the intrinsic BaTiO₃ band gap, with positions in accordance with the observed optical spectra.