Ab initio investigation of OH− vehicle migration in yttrium-doped barium zirconate

Abstract

Yttrium-doped barium zirconate is known for its good oxygen ion conductivity and excellent proton conductivity. Since a full hydration is selfdom achieved in practice, the partially hydrated material contains a significant residual fraction of oxygen vacancies which enable oxygen ion migration through a classical vacancy hopping mechanism. Effectively, this renders the material a mixed ionic conductor, with both protons and oxygen ions being mobile. Yet, the oxygen transport is often neglected since oxygen ions are expected to be far less mobile than protons within these materials. In this contribution, a combination of density functional theory calculations and kinetic Monte Carlo ensemble simulations is applied to investigate the influence of vehicle migration on the oxygen ion transport in yttrium-doped barium zirconate assuming the corporate migration of OH⁻ species. The first-principles energy calculations suggest substantially reduced migration barriers in comparison to isolated O²⁻ migration and the subsequent Monte Carlo simulations indicate an increase of the oxygen ion conductivity due to the concerted migrations, while the proton movement remains almost unaffected. Surprisingly, the inclusion of vehicle movement does not increase the oxygen ion transport directly but seems to enable oxygen vacancies trapped at yttrium defects to leave trapping regions. This greatly improves the total oxygen ion mobility in the system by classical vacancy hopping, while the direct contribution of the OH⁻ hopping to the oxygen ion conductivity seems to be detrimental.