An experimental and theoretical study of Ni impurity centers in Ba$_{0.8}$Sr$_{0.2}$TiO$_3$

TL;DR

This study combines experimental XAFS spectroscopy and theoretical calculations to investigate the local environment, charge state, and defect structure of Ni impurities in Ba$_{0.8}$Sr$_{0.2}$TiO$_3$, revealing high-spin Ni$^{2+}$ states and oxygen vacancies.

Contribution

It provides new insights into the charge state, local environment, and defect compensation mechanisms of Ni impurities in this perovskite material.

Findings

Ni has a charge state of approximately +2.5.

Ni$^{2+}$ ions are in a high-spin state at B sites.

Presence of a secondary phase BaNiO$_{3- ext{delta}}$.

Abstract

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.