Valence Flexibility and Structural Complexity in Epitaxial Cr-Based Complex Oxides

This talk will explore the epitaxial growth of Cr-based complex oxide thin films and the intricate structure-property relationships that arise in these materials. A particular emphasis will be placed on the behavior of Cr, a redox-active 3d transition metal with valence states ranging up to +6. Our research demonstrates that while Cr³⁺ is the most stable oxidation state in perovskite ABO₃ oxides, such as in LaCrO₃, higher valence states - Cr⁴⁺ and Cr⁵⁺- can be stabilized through aliovalent doping and strain tuning. We highlight the formation of vacancy-ordered superstructures, including rhombohedral SrCrO_2.8 and a brownmillerite-like phase induced by Fe doping in SrCrO_2.8. Using high-resolution STEM and EELS, we have uncovered a layer-resolved modulation of Cr valence in the brownmillerite-like structure, underscoring the remarkable ability of Cr to adapt to changes in oxygen stoichiometry. Despite Cr’s valence flexibility, challenges persist, notably the undesirable Cr compositional fluctuations in high-entropy oxide (HEO, A(5B)O₃) thin films and the formation of Cr⁶⁺ under redox-active conditions. These issues hinder their performance and use in various applications. Our findings offer valuable insights into Cr’s valence adaptability and suggest strategies to enhance the functional properties of these complex oxides.