Interpreting oxygen vacancy migration mechanisms in oxides using the layered structure motif

Abstract

The manner in which oxygen vacancies migrate in oxide ceramics is analyzed in terms of an atom layer stacking motif to elucidate similarities and differences in migration mechanisms as a function of crystal structure and chemistry. In particular, five oxide structures are examined: rocksalt, spinel, bixbyite, pyrochlore, and fluorite. While there are similarities that are related to structure in the types of migration mechanisms that are possible, there are significant differences as a consequence of chemistry and deviations from the parent structures (rocksalt and fluorite). One primary difference occurs because of structural relaxations related to the complex chemistry in spinel and pyrochlore, in which oxygen ions tend to form localized groupings within which oxygen vacancy hopping mechanisms are particularly fast. While these mechanisms cannot lead to net migration, a kinetic Monte Carlo model does show that they do influence overall diffusivity. We conclude that the atom layer stacking motif is an effective scheme to understand the dependency of oxygen vacancy migration mechanisms on oxide crystal structure.