Studying Formation of Single-Phase Cr-Mn-Fe-Co-Ni Spinel High-Entropy Oxide with Simultaneous in situ X-ray Scattering and Absorption

High-entropy oxides have emerged as a promising class of functional materials due to their tuneable compositions and resulting structural and electronic diversity. However, the synthesis of singlephase high-entropy oxides remains challenging, as the presence of multiple metals often leads to secondary phase formation. Understanding how composition and redox chemistry govern phase formation is therefore crucial for the rational design of these materials. Here, we apply rational design principles in a sol-gel synthesis to obtain a single-phase (Co0.5Ni0.5)(Cr0.33Mn0.33Fe0.33)2O4 spinel high-entropy oxide. Using quasi-simultaneous in situ X-ray scattering and X-ray absorption spectroscopy, we identify the formation of a nanostructured intermediate during synthesis. This intermediate reduces at higher temperatures to form the final high-entropy spinel phase, with Cr and Mn identified as the key redoxactive species during this transition. Modelling the extended X-ray absorption fine structure reveals the cation distribution of the final material, including the presence of Cr4+ and Mn2+ species in the tetrahedral site, which deviate from their expected oxidation states. We further show that the rare Cr4+ – typically inaccessible through commonly employed solid-state syntheses – forms in the intermediate and persists during product formation. These findings provide new insight into the redox requirements necessary for the formation of single-phase high-entropy oxides. Furthermore, we demonstrate that quasi-simultaneous in situ X-ray scattering and absorption is well-suited to probe the formation of complex materials such as HEOs, and we present, to the best of our knowledge, the first in situ structural characterization of HEO formation using this approach.