Oxygen Vacancies in High Entropy Perovskite Oxides—A Computational Study about Local Ordering, Distortion and Redox Activity

High entropy perovskite oxides (HEPOs), typically consist of four or more elements fully mixed at {A} or/and {B} sites in ABO₃ perovskite lattices. Mixing multiple cations can reach a significant configurational entropy that offers a wide, unexplored new compositional space with vast tunability to meet challenging requirements of real applications. One of applications is the two-step solar thermochemical hydrogen generation (STCH) techniques, in which HEPOs are believed to provide higher redox capacities, longer durability and lower reduction temperature.<br/><br/>Multiple cation mixing introduces new computational challenges, such as composition design rule for higher hydrogen yield, strategy to sample and analyze possible configurations, and nonstoichiometry prediction for HEPO system with spread oxygen vacancy formation energies. In this research, density functional theory (DFT) is combined with Metropolis Monte Carlo method (DFT-MC) to efficiently sample the possible cation configurations and discover the role of each {B} site elements on oxygen vacancy formation. (La_0.8Sr_0.2){MnFeCoAl}O₃ (LS_MFCA) was simulated first as one of the materials that shows good STCH performance. Cation positions or magnetic moment exchanged configurations were used in the DFT-MC simulations at 1600 K (reduction temperature in STCH process), and more than 2600 bulk, 3000 vacancy configurations that gave rise stable energies in MC simulations were saved for further analysis.<br/><br/>The neutral oxygen vacancy formation energy in LS_MFCA is sensitive to {B} site element type, charge state, local ordering and structural distortion. The oxidation state determined by magnetic moments shows that the charge states on {MnFeCoAl} are Mn⁴⁺, Fe³⁺, Co³⁺, and Al³⁺ and Co³⁺ is the predominant redox active element, which agrees with the in-situ X-ray photoelectron spectroscopy (XPS) observations. Cation mixing causes local lattice distortion. Cation Bond Valence Sum (BVS) descriptor analysis shows that Co-O bonds in bulk LS_MFCA structures are much more stretched (weakened) than Mn-O and Al-O bonds, while Fe-O bonds are slightly compressed (strengthened). Since the oxygen vacancy formation causes volume expansion in many oxides, the stretched bonds will lower the vacancy formation energy near Co. Among the saved structures, the configurations of the two B-cations connected to the oxygen (or vacancy) were analyzed for possible {B} site preference or local ordering. The elemental combination of the {B} pairs connecting to oxygen in the bulk LS_MFCA is close to random, but more than 80% of the {B} pairs connecting to oxygen vacancy contains Co. This means the local excess space due to Co³⁺-O bond stretching outpaced the tendency to generate vacancies near Mn⁴⁺, leading to the oxygen vacancy generation favoring Co site.