Paramagnetic resonance study of nickel ions in hexagonal barium titanate

X-ray diffraction patterns and electron paramagnetic resonance (EPR) powder spectra (9 and 34 GHz) of BaTiO3 + 0.04 BaO + xNiO (0.001 <= x <= 0.02) ceramics were studied to investigate the incorporation of Ni ions and their valence states as well as the development of the hexagonal phase (6H modification) of Ni-doped material with respect to doping level x and sintering temperature T-s. The 6H modification begins to occur at a nominal Ni concentration of between x = 0.005 and 0.01 and its percentage increases with increasing sintering temperature. Ni-doped BaTiO3 with x = 0.02 sintered at T-s = 1400 degrees C is completely hexagonal. In the 3C modification, present in as-sintered ceramics with low nominal Ni concentrations, only one type of Ni EPR spectrum was observed. By comparing its principal values of the g tensor with data of single-crystal measurements the clear assignment of this spectrum to Ni+ ions is possible. Two different EPR spectra with orthorhombic g tensors are observed in the as-sintered samples with hexagonal crystal structure. These spectra were assigned to Ni3+ ions with the electron spin S = 1/2 (electron configuration 3d(7), strong crystal field) substituted at Ti lattice sites corresponding to the different distorted octahedra of the hexagonal modification. Measurements of the concentration reveal that only 5% of the doping material is in the state Ni3+. No EPR spectra of Ni2+ ions have been detected in either 3C or 6H modification in as-sintered ceramics. Therefore, we suppose that the main part of nickel is substituted as Ni4+ ions on Ti4+ lattice sites. After heat treatment of the samples in H-2/Ar atmosphere a single-line spectrum with g = 2.21 +/- 0.01 at room temperature has been observed which is assigned to metallic Ni or antiferromagnetically coupled Ni2+ ions in secondary phases segregated at grain boundaries or triple points.