Vacancy-vacancy interaction and oxygen diffusion in stabilized cubic ZrO2 from first principles

Based on ab initio and classical molecular-dynamics simulations, we investigate the role of vacancy-vacancy interaction in oxygen conductivity of yttria-stabilized and scandia-stabilized cubic zirconia. It turns out that a sizable fraction of possible configurations of vacancies are local unstable, although the vacancies are at least third nearest neighbors each other. Investigation of oxygen migration pathways by ab initio metadynamics simulations show the occurrence of multiple-vacancy concerted jumps in correspondence of an unstable arrival state for a single vacancy jump. Furthermore, in the case of single vacancy jump, we have observed a strong dependence of the activation barrier on the position of the other vacancies, similar in strength to the dependence of the activation energy on the presence of nearest-neighbors yttrium ions, customarily assumed to constitute a blocking site for vacancy migration. Our results point to the need of improving existing models for oxygen diffusion in stabilized zirconia by including vacancy-vacancy interactions.