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 the Ni${}^{2+}$ and Ti${}^{4+}$ charges 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 Ni${}^{4+}$ ions at the $B$ sites.