Anelastic relaxation from hydrogen and other defects in La-doped BaTiO3

We present anelastic spectroscopy measurements between 2 and 500 K in Ba1−xLaxTiO3 (0.10 ≤ x ≤ 0.20). The phase transitions of pure BaTiO3 are shifted to lower temperature by increasing the La doping and are completely suppressed starting from x = 0.15. The samples are expected to contain H impurities since sintering was performed in flowing H2 to ensure electronic rather than cation vacancy compensation, and indeed at least three relaxation processes of the complex anelastic spectrum are strongly influenced by annealing the samples in CO or H2 at 1450 K. A thermally activated anelastic peak around 250 K (f ∼ 2 kHz) is ascribable to H hopping between different O atoms, while a possible indication of fast reorientation around a same O atom comes from a broad peak around 100 K. At least other three relaxation processes are present between 30 and 150 K, but the present data do not allow their nature to be ascertained; possible mechanisms involve polarons and hopping of Ti and possibly La among off-centre positions.