Phase stability of high entropy oxides: A critical review

After the discovery of the prototypical high-entropy oxide with formula Cu0.2Zn0.2Mg0.2Co0.2Ni0.2O, the research on these materials has shown a dramatic boost. While many studies were devoted to exploring their possible applications, only few addressed the effective reasons behind their stability. The possibility to achieve a single-phase structure is usually hastily attributed to configurational entropy. This resulted in an extensive misuse of the terms "entropy-stabilized oxides" and "high-entropy oxides", often employed as synonyms. However, the effective role of entropy in the stabilization should be demonstrated for each system, and even for Cu0.2Zn0.2Mg0.2Co0.2Ni0.2O it is still under debate. Moreover, many effects concur to the stabilization, i.e. the relative concentration, the valence state and the ionic radii of the cations involved. We will here discuss the role of configurational entropy in the phase stability, considering the main classes of crystal structures and providing a guide to critically discern between entropy and non-entropy stabilized oxides.