Activation Volume Tensor for Oxygen-Vacancy Migration in Strained CeO2 Electrolytes

We examine the effect of mechanical strain on the migration of oxygen vacancies in fluorite-structured ceria by means of density functional theory calculations. Different strain states (uniaxial, biaxial and isotropic) and strain magnitudes (up to +/- 7%) are considered. From the calculations we extract the complete activation volume tensor for oxygen-vacancy migration in CeO2, that is, all diagonal Delta V-mig,V-kk and off-diagonal Delta V-mig,V-kl tensor elements. These individual tensor elements are found, crucially, to be independent of strain state; they do, however, depend on stress (Delta V-mig,V-kk) or effective pressure (Delta V-mig,V-kl). Armed with knowledge of all tensor elements we predict strain states for which oxygen-ion transport in ceria is maximized. In general, with our approach the effect of an arbitrary strain state on the migration barrier for mass transport in a solid can be calculated quantitatively.