Low-temperature anelastic and dielectric spectroscopy measurements were performed on the perovskite ionic conductor, BaCe1−xYxO3−x/2, in the protonated, deuterated, and out-gassed states. Three main relaxation processes were attributed to proton migration, reorientation about an Y dopanted and tunnelling around the same O atom. An additional relaxation maximum appears only in the dielectric spectrum around 60K and does not involve H motion but may be of electronic origin, e.g., small polaron hopping. The peak at the lowest temperature, attributed to H tunnelling, was fitted with a relaxation rate presenting crossovers from one-phonon transitions, nearly independent of temperature, to two-phonon processes, varying as T7, to Arrhenius type. Substituting H with D lowers the overall rate by 8 times. The corresponding peak in the dielectric loss had an intensity nearly 40 times smaller than expected from the classical reorientation of the electric dipole associated with the OH complex. This fact was explained in terms of coherent tunnelling states of H in a cubic and orthorhombically distorted lattice, possibly indicating that only H in the symmetric regions of twin boundaries exhibited tunnelling, and in terms of a reduction in the effective dipole due to lattice polarization.

Hydrogen Tunneling in the Perovskite Ionic Conductor BaCe1−xYxO3−δ. F.Cordero, F.Craciun, F.Deganello, V.La Parola, E.Roncari, A.Sanson: Physical Review B, 2008, 78[5], 054108 (9pp)