Hydrogen tunneling in the perovskite ionic conductor BaCe1-xYxO3-d

We present low-temperature anelastic and dielectric spectroscopy measurements on the perovskite ionic conductor BaCe1-xYxO3-x/2 in the protonated, deuterated, and outgassed states. Three main relaxation processes are ascribed to proton migration, reorientation about an Y dopant, and tunneling around the same O atom. An additional relaxation maximum appears only in the dielectric spectrum around 60 K and does not involve H motion but may be of electronic origin, e.g., small polaron hopping. The peak at the lowest temperature, assigned to H tunneling, has been 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 eight times. The corresponding peak in the dielectric loss has an intensity nearly 40 times smaller than expected from the classical reorientation of the electric dipole associated with the OH complex. This fact is discussed in terms of coherent tunneling states of H in a cubic and orthorhombically distorted lattice, possibly indicating that only H in the symmetric regions of twin boundaries exhibit tunneling, and in terms of reduction in the effective dipole due to lattice polarization.