An experimental and theoretical study of Ni impurity centers in Ba0.8Sr0.2TiO3

Abstract

The local environment and the charge state of a nickel impurity in cubic Ba_0.8Sr_0.2TiO₃ 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 Ni²⁺ ions are in a high-spin state at the B sites of the perovskite structure and the difference of charges of Ni²⁺ and Ti⁴⁺ is mainly compensated by distant oxygen vacancies. In addition, a considerable amount of nickel in the sample is in a second phase BaNiO_{3 − δ}. 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 Ni⁴⁺ ions at the B site.