Tuning Surface Structure and Electronic Properties of High Entropy Oxide Thin Films for Water Oxidation

High-entropy perovskite oxides (HEPOs) expand the versatility of conventional ABO₃-type perovskites by incorporating multiple equi-atomic A- and/or B-site cations, unlocking exceptional compositional complexity and functional tunability. However, it remains unclear how large physicochemical differences among these cations—differences in ionic radii, charge, redox chemistry—govern their response to substrate-imposed strain and to oxygen chemical potential, thereby shaping atom arrangements and properties. In this talk, I will use epitaxial B-site HEPO La(5B)O₃ (5B = Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2) thin films as a model to show how we can understand, predict, and ultimately control the formation and transformation of metastable configurations (e.g., cation segregation, local ordering, and composition gradients). I will show that under high oxygen partial pressure during PLD, La(5B)O₃ films exhibit obvious Cr segregation—depletion at the interface and enrichment at the surface. Layered-resolved EELS indicates that this segregation arises from oxidation-induced migration of smaller, high-valence Cr cations during growth. Systematically lowering the oxygen partial pressure during growth suppresses the Cr segregation. A combination of microscopy, spectroscopy, and electrochemical measurements establishes a quantitative link between defect chemistry and electrocatalytic activity, providing design rules for coupling strain and defect control to predict more robust HEPO electrocatalysts.