A Multi-Pronged Approach to the Determination of Cation Distribution, Site Selectivity, and Chemical Ordering in “High Entropy” Spinel Oxides

"High entropy" oxides (HEOs) are complex, multifunctional materials that have provided a paradigm shift on the design of technologically important materials. With their high number of chemical substituents, they offer a wide chemical space to explore compared with traditional solid solutions and common semiconductors. They also exhibit magnetic uniformity seldom associated with highly disordered materials. The versatility of HEOs and their excellent emergent properties have made these materials potential candidates for use as anodes in lithium-ion batteries, electrocatalysts in water splitting, and spintronic materials. However, the compositional complexity of these materials makes them challenging to understand at a fundamental level. This work represents a milestone in our understanding of "high entropy" materials, owing to our use of a multipronged approach: high-resolution synchrotron X-ray diffraction, resonant X-ray diffraction, X-ray absorption spectroscopy, and X-ray and neutron total scattering techniques. Our comprehensive characterization reveals key insights regarding the site selectivity and chemical ordering in a "high entropy" spinel oxide fabricated via a conventional solid-state reaction. Cr, Ni, and Mn have been found to strongly prefer an octahedral environment, while Co has been shown to predominantly occupy tetrahedral sites. Evidence of Ni clustering is reported for the first time.