An experimental and theoretical study of Ni impurity centers in Ba0.8Sr0.2TiO3
The local environment and the charge state of a nickel impurity in cubic Ba0.8Sr0.2TiO3 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 BaNiO3 − 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.
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