Structural position and charge state of nickel in SrTiO3

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 O₃ solid solution and NiTiO₃ which can coexist. According to the EXAFS data, in the single-phase SrTi_0.97Ni_0.03O₃ 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 NiTiO₃ is 2+, whereas in the SrTi_{1 ? x} Ni _x O₃ 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 O₃ solid solution. This doping seems to be the most promising for solar energy converters based on the bulk photovoltaic effect.