Valence change and phase stability of3d-doped BaTiO3annealed in oxygen and hydrogen

BaTi${\mathrm{O}}_3$ ceramics doped with several iron-group elements up to 2.0 mole% were identically prepared and afterwards annealed at 700°C in ${\mathrm{O}}_2$ at 150 bar or in ${\mathrm{H}}_2$-${\mathrm{H}}_2$O with an oxygen partial pressure of ${10}^{-22\mathrm{}}$ bar. The valence states of the dopants were determined by magnetic-susceptibility measurements, and the phase-transition temperatures were established by dielectric measurements. The transition temperatures and the charge of the Cr, Mn, and Co dopants were found to change reversibly upon annealing at different oxygen partial pressures whereas both remained constant for Fe, Ni, Zn, and Ga dopants. The change of the concentration of oxygen vacancies, which proved to be inherently connected with valence changes, was investigated gravimetrically.