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 (ΔH_mig,v = 1.2-2.4 eV) are substantially higher than those predicted in computational work (ΔH_mig,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 (D_v) in orthorhombic LaMnO₃. Our results indicate that the presence of cation vacancies can bring the computational values of ΔH_mig,v into good agreement with experimental data. Applying an analytical model to our results, we predict that isothermal values of D_v 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.