Rational synthesis and structure control of high-entropy spinel oxides

High entropy oxides (HEO) 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 co-crystallization of thermodynamically more stable phases. Here, we report a facile sol-gel based synthesis and design approach for the formation of single phase M 3 O 4 spinel oxides, containing M = Cr, Mn, Fe, Co, Ni, and Cu in various compositions. For 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 on 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.