The surprisingly high activation barrier for oxygen-vacancy migration in oxygen-excess manganite perovskites

The current description of oxygen-vacancy behaviour in (La,Sr)MnO3 perovskite-type oxides, though widely accepted, ignores a pronounced discrepancy. Values of the activation enthalpy of oxygen-vacancy migration reported in experimental investigations (ΔHmig,v = 1.2-2.4 eV) are substantially higher than those predicted in computational work (ΔHmig,v = 0.5-1.0 eV). In this study we examine the origin of this discrepancy using molecular-dynamics simulations. Specifically we investigate the effect of various cation defects (Sr substituents, La vacancies, Mn vacancies, both types of cation antisites) on the diffusivity of oxygen vacancies (Dv) in orthorhombic LaMnO3. Our results indicate that the presence of cation vacancies can bring the computational values of ΔHmig,v into good agreement with experimental data. Applying an analytical model to our results, we predict that isothermal values of Dv in manganite perovskites containing cation vacancies will depend strongly on oxygen partial pressure (contrary to the standard assumption). The implications of our results for modelling the point-defect chemistry of, and oxygen diffusion in, manganite perovskites are discussed.