Dynamics of Hydride Ions in Metal Hydride-Reduced BaTiO ₃ Samples Investigated with Quasielastic Neutron Scattering

Perovskite-type oxyhydrides, BaTiO _3-x H _x , have been recently shown to exhibit hydride-ion (H ^- ) conductivity at elevated temperatures, but the underlying mechanism of hydride-ion conduction and how it depends on temperature and oxygen vacancy concentration remains unclear. Here, we investigate, through the use of quasielastic neutron scattering techniques, the nature of the hydride-ion dynamics in three metal hydride-reduced BaTiO ₃ samples that are characterized by the simultaneous presence of hydride ions and oxygen vacancies. Measurements of elastic fixed window scans upon heating reveal the presence of quasielastic scattering due to hydride-ion dynamics for temperatures above ca. 200 K. Analyses of quasielastic spectra measured at low (225 and 250 K) and high (400-700 K) temperature show that the dynamics can be adequately described by established models of jump diffusion. At low temperature, ≤250 K, all of the models feature a characteristic jump distance of about 2.8 Å, thus of the order of the distance between neighboring oxygen atoms or oxygen vacancies of the perovskite lattice and a mean residence time between successive jumps of the order of 0.1 ns. At higher temperatures, >400 K, the jump distance increases to about 4 Å, thus of the order of the distance between next-nearest neighboring oxygen atoms or oxygen vacancies, with a mean residence time of the order of picoseconds. A diffusion constant D was computed from the data measured at low and high temperatures, respectively, and takes on values of about 0.4 × 10 ^-6 cm ^-2 s ^-1 at the lowest applied temperature of 225 K and between ca. 20 × 10 ^-6 and 100 × 10 ^-6 cm ^-2 s ^-1 at temperatures between 400 and 700 K. Activation energies E _a were derived from the measurements at high temperatures and take on values of about 0.1 eV and show a slight increase with increasing oxygen vacancy concentration.