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

Abstract

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.

Mg4InSbO8 Spinel: Double 2:1 Ordering in Tetrahedral and Octahedral Sublattices and Perspective Application as an Optical Thermometer via Mn2+/4+ Doping

“…Spinel-type oxides with one of the classic mineral structures have attracted tremendous interest in the field of solid-state chemistry and physics due to their abundant and intriguing physical properties, such as multiferroics, geometrically frustrated magnetism, and spin and orbital ordering. − Spinel oxides have also shown great potential in electrocatalytic oxygen evolution and energy storage for cathode materials. − Very recently, spinel oxides were also reported to be a good platform for high-entropy oxides to explore correlations between configurational entropy, site selectivity, and magnetism. − All of these appealing properties of spinel oxides are strongly related to the unique crystal structure comprising multiple cation sites with different coordination geometry. − Simple spinel oxides usually have a general formula of AB 2 O 4 or AB′B″O 4 , where both the tetrahedrally coordinated A-site and octahedrally coordinated B-site can accommodate cations with a wide range of ionic radius and charge, such as Li + , Zn 2+ , Mg 2+ , 3d TMs, Al 3+ /Ga 3+ /In 3+ , Ge 4+ /Sn 4+ , Sb 5+ , and Te 6+ .…”

Platinum on High-Entropy Aluminate Spinels as Thermally Stable CO Oxidation Catalysts

“…(CoCuMgNi)0.8Al2O3.8 compositions [20]. This latter effect suppresses the formation temperature of complex oxides [12,19]. The PXRD patterns of prepared Pt-loaded spinel samples are shown in Figure 1.…”

“…Spinel samples were first evaluated for phase purity via PXRD. The synthesis of phase pure aluminate spinels required varying calcination temperature depending on spinel composition, as discussed in our previous work [19]. These calcination temperatures are listed in Table 1.…”

“…Spinel samples were synthesized according to our previously reported sol-gel synthesis [19]. A total of 5 g of polyvinylpyrrolidone (PVP, Sigma Aldrich, Milwaukee, WI, USA, average molecular weight = 40,000) was dissolved into 100 mL of deionized water while stirring.…”

“…In this study, we evaluate Pt on HEO aluminate spinels as compositionally and structurally novel catalysts for the oxidation of CO, one of the primary chemical reactions facilitated by automotive aftertreatment catalysts. Our recent work demonstrated the stabilization of stoichiometric and non-stoichiometric high-entropy aluminate spinels at 800 • C and higher temperatures, which thermally degrade conventional catalysts [19]. Here, we add Pt to simple and high-entropy spinel supports and compare catalytic activity before and after thermal aging.…”