Understanding Cation Distribution in Nominal High-Entropy Oxide Spinels: An Experimental Study

Multicomponent oxides with a spinel structure, also referred to as high-entropy oxide spinels, are emerging functional materials with a wide range of potential applications, including magnetic devices and energy-conversion technologies. However, one of the main challenges in their development is the lack of general guidelines governing cationic occupancy, which critically determines their physical and chemical properties. To address this issue, we performed a comprehensive investigation of two series of spinel materials with nominal composition Mx(Cr,Fe,Mn,Ni)3–xO4 (x = 0, 0.33, 0.5, 0.6, 0.82, 1), where Co2+ and Al3+ were used as substituting cations with distinctly different properties. Combined X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements enabled a detailed elucidation of the atomic-scale cation distribution. Despite the pronounced chemical complexity, the cation distributions were found to be strongly correlated with site preferences governed by crystal field stabilization energies, leading to the following site occupations: Co2+ predominantly tetrahedral, Ni2+ octahedral, Fe3+ mainly tetrahedral, Cr3+ and Al3+ octahedral. Manganese exhibits a strong tendency toward multivalency, occurring as Mn2+/Mn3+/Mn4+ on both sublattices and acting as the primary charge-compensation mechanism in both systems. Overall, the results demonstrate that, despite their multicationic nature, the investigated spinels follow similar cation-distribution rules as conventional spinel oxides. These findings provide important insights for the rational design of spinel-type high-entropy oxides tailored for specific applications.