An experimental and theoretical study of Ni impurity centers in Ba$_{0.8}$Sr$_{0.2}$TiO$_{3}$

The local environment and the charge state of a nickel impurity in cubic Ba$_{0.8}$Sr$_{0.2}$TiO$_{3}$ are studied by XAFS spectroscopy. According to the XANES data, the mean Ni charge state is $\sim$2.5+. An analysis of the EXAFS spectra and their comparison with the results of first-principle calculations of the defect geometry suggest that Ni$^{2+}$ ions are in a high-spin state at the $B$ sites of the perovskite structure and the difference of charges of Ni$^{2+}$ and Ti$^{4+}$ is mainly compensated by distant oxygen vacancies. In addition, a considerable amount of nickel in the sample is in a second phase BaNiO$_{3-\delta}$. 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 Ti$^{4+}$ ions at the $B$ site.