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.