Assessing Phase Stability in High-Entropy Materials by Design of Experiments: The Case of the (Mg,Ni,Co,Cu,Zn)O System

In this study, we aimed to explore the phase stability of high-entropy oxides (HEOs) beyond their conventional equimolar composition, which presents the maximum configurational entropy. This task is challenging due to the large number of compositional parameters involved. We used the design of experiments as a strategy to investigate the compositional range of stability of the rock salt (RS) structure in the (Mg,Ni,Co,Zn,Cu)O quinary system, featuring the prototypical HEO Mg0.2Ni0.2Co0.2Zn0.2Cu0.2O. Our study revealed that the chemical nature of the RS-native oxides (NiO, MgO, and CoO) significantly affects the phase stability of the RS-HEO, suggesting that the HEO stability is not solely governed by the balance of configurational entropy and enthalpy of mixing. In addition, a single high-entropy phase can be achieved on a wide out-of-equimolar set of compositions, thereby broadening the compositional range that should be explored in the search for innovative materials with unique properties and applications.