Ab initio and empirical defect modeling of LaMnO3±δ for solid oxidefuel cell cathodes

Abstract

Sr doped LaMnO₃ is a perovskite widely used for solid oxide fuel cell (SOFC) cathodes. Therefore, there is significant interest in its defect chemistry. However, due to coupling of defect reactions and inadequate constraints of the defect reaction equilibrium constants obtained from thermogravimetry analysis, large discrepancies (up to 4 eV) exist in the literature for defect energetics for Sr doped LaMnO₃. In this work we demonstrate how ab initio energetics and empirical modelling can be combined to develop a defect model for LaMnO₃. Defect formation enthalpies, including concentration dependence due to defect interactions, are extracted from ab initio energies calculated at various defect concentrations. Defect formation entropies for the defect reactions in LaMnO₃ involving O²⁻(solid) ↔ ½O₂(gas) + 2e⁻ are shown to be accessible through combining the gas phase thermodynamics and simple models for the solid phase vibrational contributions. This simple treatment introduces a useful constraint on fitting defect formation entropies. The predicted defect concentrations from the model show good agreement with experimental oxygen nonstoichiometry vs.P(O₂) for a wide range of temperatures (T = 873–1473 K), suggesting the effectiveness of the ab initio defect energetics in describing the defect chemistry of LaMnO₃. Further incorporating a temperature dependent charge disproportionation energy within 0.0–0.2 eV, the model is capable of describing both defect chemistry and oxygen tracer diffusivity of LaMnO₃. The model suggests an important role for defect interactions which are typically excluded from LaMnO₃ defect models, and sensitivity of the oxygen defect concentration to the charge disproportionation energy in the high P(O₂) region. Similar approaches to those used here can be used to model the defect chemistry for other complex oxides.