An Experimental and Theoretical Study of Ni Impurity Centers in [Ba.sub.0.8][Sr.sub.0.2]Ti[O.sub.3]

The local environment and the charge state of a nickel impurity in cubic [Ba.sub.0.8][Sr.sub.0.2]Ti[O.sub.3] are studied by XAFS spectroscopy. According to the XANES data, the mean Ni charge state is ~2.5+. An analysis of the EXAFS spectra and their comparison with the results of first-principle calculations of the defect geometry suggest that [Ni.sup.2+] ions are in a high-spin state at the B sites of the perovskite structure and the difference of charges of [Ni.sup.2+] and [Ti.sup.4+] is mainly compensated by distant oxygen vacancies. In addition, a considerable amount of nickel in the sample is in a second phase BaNi[O.sub.3-[delta]]. The measurements of the lattice parameter show a decrease in the unit cell volume upon doping, which can indicate the existence of a small amount of [Ni.sup.4+] ions at the B site.