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The article focuses on the synthesis and characterization of a high-entropy oxide (HEO) called Iron(II) Oxide (FeO)-HEO, which contains divalent Manganese (Mn) and Iron (Fe) in a rocksalt crystal structure. Topics include the concept of HEOs and their unique compositional features, the challenge of introducing divalent Mn(II) and Fe(II) into a rocksalt HEO, and the phase stability of FeO-HEO under different conditions.

High-entropy oxides (HEOs) have aroused growing interest due to fundamental questions relating to their structure formation, phase stability, and the interplay between configurational disorder and physical and chemical properties. Introducing Fe(II) and Mn(II) into a rocksalt HEO is considered challenging, as theoretical analysis suggests that they are unstable in this structure under ambient conditions. Here, we develop a bottom-up method for synthesizing Mn- and Fe-containing rocksalt HEO (FeO-HEO). We present a comprehensive investigation of its crystal structure and the random cation-site occupancy. We show the improved structural robustness of this FeO-HEO and verify the viability of an oxygen sublattice as a buffer layer. Compositional analysis reveals the valence and spin state of the iron species. We further report the antiferromagnetic order of this FeO-HEO below the transition temperature ~218 K and predict the conditions of phase stability of Mn- and Fe-containing HEOs. Our results provide fresh insights into the design and property tailoring of emerging classes of HEOs.