Beyond configurational entropy: the role of solubility equilibria in the stability of the system (Co,Cu,Mg,Ni,Zn)O

High entropy oxides are a novel class of materials, where multiple cations can be incorporated in a single-phase structure. Since the discovery of the prototypical compound Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2O, the research on these materials has shown an impressive boost. This compound adopts a rock-salt structure, even if CuO and ZnO are usually stable as tenorite and wurtzite. The attainment of a single phase is usually ascribed to the substantial value of configurational entropy that counterbalances unfavourable enthalpy terms. Here, we reconsider the effective role of configurational entropy in the stabilization in view of the solubility and redox equilibria involved. A careful examination of the high-temperature solubility limit of CuO in each native rock salt oxide proves to be useful to predict the phase stability. However, when the number of components is high, the behaviour becomes, at least to some extent, distinct from that of the constituent rock-salt oxides and difficult to predict.