Since the discovery by Rost et al. of Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O as the first high entropy oxide (HEO) with rock salt (RS) structure, [1] the research on HEOs has been attracting increasing attention for different applications, such as Li-ion batteries, catalysis, etc.  It is generally accepted that the HEO can be obtained only with an equimolecular amount of the cations, when the configurational entropy reaches its maximum. To investigate this point we performed a systematic study of HEOs in the (Mg,Co,Ni,Cu,Zn)O family, unveiling the role of temperature and chemical composition in determining the stability of the RS phase. The results bring to question the concept that the HEO in the (Mg, Co, Ni, Cu, Zn)O system does form only because of a configurational entropy contribution. In fact, single-phase solid solutions (SS) can be obtained moving from a 5- to a 2-component oxide as long as the molar fractions of ZnO and CuO are kept below a limiting value close to 0.2, which is dictated by the high-temperature solubility equilibria. Therefore, single-phase SSs can still be obtained by reducing the configurational entropy from 1.61R to 0.5R. We also demonstrate that the tendency of these SS’s rock-salt structures to distort from the cubic symmetry decreases with the configurational entropy, suggesting that the configurational entropy does not have a central role as stabilizing factor of the RS structure.
Then, we investigated the stability range of the HEO as a function of the chemical composition. Dealing with 5 cations, a systematic investigation would require a huge number of experiments. We tackled this problem using a chemometric approach. Using the experimental design we build a model for the fraction of RS phase in the entire compositional range investigating less than 50 samples. This allowed to identify a fairly large compositional domain of stability for the RS phase, which appears to be even larger for the Ni-rich ternary compositions than for the 5-element SS. Again, this confirms the limiting role of entropy in stabilizing the (MgCoNiCuZn)O system.
Eventually, we are developing an experimental high-throughput setup to speed up and automatize the synthesis and the characterization of these complex systems, in order to apply the same chemometric approach to other high entropy ceramics

[1] C. Rost, E. Sachet, T. Borman, A. Moballegh, E. C. Dickey, E. C., D. Hou, J. L. Jones, S. Curtarolo, J.-P Maria. Nat. Commun. 6, (2015) 8485.
[2] M. Fracchia, M. Coduri, M. Manzoli, P. Ghigna, U. Anselmi Tamburini. Nature Communications 13 (1), (2022), 2977.