Entropy-driven phase transitions in complex ceramic oxides

The recent development of multicomponent, high-entropy oxides (HEOs) has sparked research into ceramics with significant compositional and structural diversity. The wide range of compositions and structures achievable with these HEOs presents a promising research opportunity, as these materials may be custom-designed to serve in several important applications, from chemical catalysis to lithium-ion batteries. Many of the unique properties which make these materials viable for a variety of applications are attributed to the characteristic single-phase (SP) structure of HEOs. As such, some researchers of HEOs have focused on understanding the driving force which results in this transition from a multiphase to a SP structure. The continued development of these materials relies on establishing a sound fundamental understanding of these critical phase transformations. To that end, in this review, we will go over the recent research advancements investigating phase transitions in HEOs. In this review, we will leverage mathematical and empirical evidence to compare the effects of enthalpic and entropic contributions to the free energy of mixing and, in turn, their impact on the transition to SP, HEOs. Emphasis will be placed on the role of entropy stabilization in these phase transitions as well as other property effects arising from high-entropic lattice disorder.