Oxygen Vacancy Defect Migration in Titanate Perovskite Surfaces: Effect of the A‑Site Cations

Oxygen vacancy formation energies and migration barriers in (001) surfaces of CaTiO₃, SrTiO₃, and BaTiO₃ have been investigated using first principles density functional theory. The degree of distortion within the TiO₂ sublattice in the presence of defects and consequently the defect formation energies in these titanate surfaces are determined by the size of the A-site cation (Ca²⁺ < Sr²⁺ < Ba²⁺). This is notably the case for CaTiO₃, in which the presence of a vacancy defect leads to a heavily distorted local orthorhombic structure within the (001) slab depending on the defect position, despite the overall cubic symmetry of the material modelled. This effectively leads to the TiO₂ sublattice acting as a thermodynamic trap for oxygen vacancy defects in CaTiO₃. By contrast, calculated vacancy diffusion pathways in SrTiO₃ and BaTiO₃ indicate that vacancy diffusion with these larger A-site cations is kinetically and not thermodynamically controlled.