Hydrogen tunneling in the perovskite ionic conductor BaCe(1-x)Y(x)O(3-d)

TL;DR

This study investigates proton tunneling in the perovskite ionic conductor BaCe(1-x)Y(x)O(3-x/2) using low-temperature spectroscopy, revealing quantum tunneling behavior and its dependence on lattice structure and isotope substitution.

Contribution

It provides the first detailed analysis of hydrogen tunneling mechanisms in this perovskite, including the effects of lattice distortion and isotope substitution on tunneling rates.

Findings

Proton tunneling exhibits crossovers from one-phonon to two-phonon processes.

Replacing H with D reduces tunneling rate by a factor of 8.

Tunneling is likely confined to symmetric regions of twin boundaries.

Abstract

We present low-temperature anelastic and dielectric spectroscopy measurements on the perovskite ionic conductor BaCe(1-x)Y(x)O(3-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 a 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 T^7, to Arrhenius-like. Substituting H with D lowers the overall rate by 8 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 of the effective dipole due to lattice polarization.