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

• Published in: Physical chemistry chemical physics : PCCP 2020-07, Vol.22 (25), p.14329-14339
• Main Authors: Börgers, Jacqueline M, De Souza, Roger A
• Format: Article
• Language: English
• Subjects: Activation; Cations; Computer simulation; Enthalpy; Lanthanum compounds; Manganites; Mathematical models; Molecular dynamics; Oxygen; Partial pressure; Perovskites; Strontium; Vacancies
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d0cp01281e

The current description of oxygen-vacancy behaviour in (La,Sr)MnO 3 perovskite-type oxides, though widely accepted, ignores a pronounced discrepancy. Values of the activation enthalpy of oxygen-vacancy migration reported in experimental investigations (Δ H mig,v = 1.2-2.4 eV) are substantially higher than those predicted in computational work (Δ H 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 LaMnO 3 . Our results indicate that the presence of cation vacancies can bring the computational values of Δ H 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. Cation vacancies diminish the oxygen-vacancy diffusivity, raise the activation enthalpy, and cause the diffusivity to depend on oxygen activity.