Is Configurational Entropy the Main Stabilizing Term in Rock-Salt Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O High-Entropy Oxide?

We question the conclusions reported in the paper "Entropy-stabilized Oxides, by C. Rost et al., by looking into the role of configurational entropy as the stabilization of the rock-salt cubic structure of the Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O (HEO) solid solution (SS). First, we demonstrate that configurational entropy can be reduced from 1.61R for HEO to 0.5R for a two-member SS, still obtaining a single-phase material if the molar fractions of ZnO and CuO are 0.2. These SSs behave identically as HEO regarding the reversible transformation between a multi- and single-phase states when temperatures are cycled between 800 and 1000 °C. Second, we demonstrate that the different SSs presenting a configurational entropy significantly lower than HEO, are less prone to the cubic to tetragonal structural distortion, suggesting that the configurational entropy has not the central role as stabilizing factor of the rock-salt structure.