Vacancy-Driven Stabilization of Sub-Stoichiometric Aluminate Spinel High Entropy Oxides

Despite significant recent developments in the field of high entropy oxides, previously reported HEOs are overwhelmingly stoichiometric structures containing a single cationic site and are stabilized solely by intermixing increasing numbers of cations. For the first time, we demonstrate here that cationic vacancies can significantly increase configurational entropy and stabilize phase-pure HEOs. Aluminate spinel HEOs with AB2O4 stoichiometry are used as a model crystal structure. These spinels tolerate large divalent cation deficiencies without changing phase, allowing for high concentrations of cationic vacancies. Stoichiometric and sub-stoichiometric spinels (with A:B molar ratios <0.5), which contained various mixtures of Co, Cu, Mg, Mn, Ni, and cationic vacancies in nominal equimolar concentration, were systematically compared as a function of heat treatment temperature and number of unique cationic species. We found that the same number of cationic species were needed to stabilize both stoichiometric and sub-stoichiometric nickel-containing spinels at 800 °C calcination, as exemplified by (CoCuMgNi)Al2O4 and (CoMgNi)0.75Al2Ox samples, signifying that vacancies stabilize phase-pure spinels similarly to cations. The chromatic, structural, and chemical properties of these complex spinels were highly tunable via incorporation of cationic vacancies and multiple divalent metals, promoting their potential application as unique pigments, catalysts, and thermal coatings.