A statistical understanding of oxygen vacancies in distorted high-entropy oxides

Abstract: High entropy perovskite oxides (HEPOs) have emerged as a promising family of stable electrode materials for high-temperature water splitting. The concentration of oxygen vacancies in HEPOs significantly influences critical properties, such as the ionic conductivity and thermal expansion coefficient. However, predicting oxygen vacancy concentrations for the design of HEPOs with high-entropic A-sites remains challenging due to the complex arrangements of metal cations. Our experimental findings reveal that variations in A-site cation size constitute a crucial factor in affecting the oxygen vacancy concentration. High-throughput atomistic simulations using a machine-learned universal interatomic potential reveal that the lattice distortions from A-site size variations result in a broadened distribution of oxygen vacancy formation energies. Our theoretical analysis, grounded in statistical thermodynamics, provides formulations for the enthalpy and entropy of oxygen vacancy formation as functions of variance in oxygen bonding energy. Altogether, our results show mixing metal cations of varying sizes in the A-site creates statistical effects on the oxygen vacancy thermodynamics, resulting in a unique approach to tuning their concentrations for complex perovskite oxides.