Rational Synthesis and Structure Control of High-Entropy Spinel Oxides

Abstract High-entropy oxides (HEOs) are a new class of complex solid solutions, containing five or more distinct metal cations in a single crystallographic site. HEO spinels are promising for a wide range of energy applications, such as catalysis and magnetic applications due to their high chemical flexibility. However, achieving single-phase formation and full ion incorporation into the spinel structure is often challenging due to cocrystallization of thermodynamically more stable phases. Here, we report a facile sol–gel-based synthesis and design approach for the formation of single-phase M3O4 spinel oxides, containing M = Cr, Mn, Fe, Co, Ni, and Cu in various compositions. For an in-depth structural analysis of the samples, we used X-ray diffraction and total scattering with Rietveld and pair distribution function refinements. X-ray absorption spectroscopy provided information about the oxidation states of the metals. By systematically investigating five-component equimolar systems, we find that phase segregation into either CuO, rocksalt, and/or different spinels occurs when Co, Cu, and Ni are simultaneously coexisting in the composition. The phase segregation is addressed by considering the electronic-state preferences of the respective metals. Using a relatively simple mixing rule, we adjusted the previously segregated compositions to form single-phase complex solid solutions containing the desired principal elements. We further demonstrate great control over the synthesis process; as an example, we can finely control the Cu+/2+ ratio through compositional changes and crystallite sizes from 4 to 50 nm by calcination temperature. This work not only contributes to the understanding of successful HEO spinel synthesis, but we anticipate that our material design approach can also be extended to other high-entropy oxide systems.