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

Despite significant recent developmentsin the field of high entropyoxides, previously reported HEOs are overwhelmingly stoichiometricstructures containing a single cationic site and are stabilized solelyby intermixing increasing numbers of cations. For the first time,we demonstrate here that cationic vacancies can significantly increaseconfigurational entropy and stabilize phase-pure HEOs. Aluminate spinelHEOs with AB(2)O(4) stoichiometry are used as amodel crystal structure. These spinels tolerate large divalent cationdeficiencies without changing phase, allowing for high concentrationsof cationic vacancies. Stoichiometric and sub-stoichiometric spinels(with A:B molar ratios <0.5), which contained various mixturesof Co, Cu, Mg, Mn, Ni, and cationic vacancies in nominal equimolarconcentration, were systematically compared as a function of heattreatment temperature and number of unique cationic species. We foundthat the same number of cationic species were needed to stabilizeboth stoichiometric and sub-stoichiometric nickel-containing spinelsat 800 degrees C calcination, as exemplified by (CoCuMgNi)Al2O4 and (CoMgNi)(0.75)Al2Ox samples, signifying that vacancies stabilize phase-pure spinelssimilarly to cations. The chromatic, structural, and chemical propertiesof these complex spinels were highly tunable via incorporation ofcationic vacancies and multiple divalent metals, promoting their potentialapplication as unique pigments, catalysts, and thermal coatings.