Entropy Engineering in the Spinel High Entropy Oxide

The discovery and development of high entropy oxides (HEOs) has garnered significant attention, in no small part due to the enhancement of functional properties that they often present. As an atomistic explanation of HEOs is beyond current theoretical techniques, auxiliary “descriptors” that characterize them are invaluable. Configurational entropy, being a measure of disorder, is considered a key descriptor when addressing the properties of HEOs. Notably, it has been demonstrated that in systems with inequivalent sublattices, crystal field effects can overwhelm and suppress configurational entropy, introducing a degree of site selectivity in the cationic distribution [1,2]. However, most studies on the subject treat configurational entropy as a fixed, static quantity. <br/> <br/>Here, we address this matter within the framework of the 3d transition metal-based AB₂O₄ spinel-type HEO (Cr,Mn,Fe,Co,Ni)_3-xGa_xO₄ system, an ideal setting that provides octahedral and tetrahedral sublattices with strong crystal field effects. We harness x-ray absorption and magnetic circular dichroism to elucidate the spinel HEO’s site selectivity and quantify its configurational entropy. Our analysis shows that Ga substitution provides an avenue to seamlessly tune the octahedral sublattice from the medium entropy to the high entropy regime, with the tetrahedral sublattice going from low entropy to medium entropy. These results imply that through careful cation selection, the two sublattices of the spinel HEO can be precisely tuned across different entropy regimes, demonstrating the principle of entropy engineering in HEOs [2].<br/> <br/>We then briefly explore the prospect of another avenue through entropy engineering in HEOs. It is well established that changes to the processing conditions involved in the growth of binary spinel oxides can have striking effects on their degree of inversion and the materials’ final properties [3,4]. This tunability hints at the possibility of extending the principles of entropy engineering to other. Our study then turns to the fully 3d (Cr,Mn,Fe,Co,Ni)₃O₄ spinel HEO, where several synthesis routes and their effect on the final materials’ site selectivity are discussed, hinting at the richness of the energy landscape in the spinel HEO. <br/> <br/> <br/>[1] Sarkar et al., Acta Materialia, 2022<br/>[2] <b>Johnstone*, González-Rivas*, Hallas</b>, et al., accepted in Journal of the American Chemical Society, 2022<br/>[3] Dronova et al., Proceedings of the National Academy of Sciences, 2022<br/>[4] Venturini et al., Journal of Magnetism and Magnetic Materials, 2019