Structural position and charge state of nickel in SrTiO$_3$

The properties of nickel-doped strontium titanate are studied using X-ray diffraction and XAFS spectroscopy. It is shown that, independently of preparation conditions, the most stable phases in the samples are single-phase SrTi$_{1-x}$Ni$_x$O$_3$ solid solution and NiTiO$_3$ which can coexist. According to the EXAFS data, in the single-phase SrTi$_{0.97}$Ni$_{0.03}$O$_3$ sample the nickel atoms substitute the titanium atoms and are on-center ones. In this case, no distortions of the oxygen octahedron which would appear in the presence of oxygen vacancies in the nickel environment were detected. An analysis of the XANES spectra shows that the nickel charge state in NiTiO$_3$ is 2+, whereas in the SrTi$_{1-x}$Ni$_x$O$_3$ solid solution it is close to 4+. It is shown that the strongest light absorption in doped samples is associated with the presence of tetravalent Ni in the SrTi$_{1-x}$Ni$_x$O$_3$ solid solution. This doping seems to be the most promising for solar energy converters based on the bulk photovoltaic effect.