Proton Mobility in the In-Doped CaZrO3 Perovskite Oxide

Abstract

First-principles calculations, based on density functional theory, are exploited to investigate the mechanisms and energetics of proton mobility in CaZrO3. The computations accurately reproduce the observed orthorhombic crystal structure of the material. Proton binding sites in the lattice are determined for a range of In dopant concentrations and the corresponding binding strengths are given on a relative energy scale. A proton is typically found to be more strongly bound by 0.1-0.2 eV to the sites at In octahedra than to equivalent sites far from the dopant, though binding energies for certain sites remote from the dopant can exceed those on adjacent octahedra. This suggests that dopant-proton trapping is relatively weak and short-ranged. A series of constrained optimizations is carried out to evaluate minimumenergy paths between the binding sites. A set of proton-transfer jumps and reorientations is identified and associated energy barriers for these proton steps are calculated. It is found that interoxygen transfer and rotation of the proton about a single oxygen have comparable barrier heights. The calculated lowest energy paths that lead to proton propagation in CaZrO3 exhibit energy barriers in excess of 0.6 eV. Lattice dynamics calculations are used to evaluate the normal modes relevant for proton mobility and the associated attempt frequencies. In this manner, a comprehensive set of data is provided from which the rates of proton migration in In:CaZrO3 may be determined.