Control of High Entropy Oxide Formation

Much of recent materials discovery has focused on "high entropy" systems with multiple components on the lattice sites. There has been excitement in both the metals and ceramics communities around new properties and the reduction of expensive and rare elements. The basic principle revolves around the use of multiple species that generate a high entropy of mixing that can counteract the enthalpic penalties associated with mixing different species. There are now a large number of examples of successfully produced compounds in a variety of crystal structures for both ceramic and metal systems. Despite the large amount of data now in the literature, our understanding of the mixing and formation of these materials is still relatively poor.<br/><br/>The perovskite lattice is a particularly exciting opportunity as mixing can be carried out on both the A or B site presenting a wealth of enthalpic and entropic possibilities to achieve a single-phase solid solution with a favourable Gibbs free energy. We use this crystal structure as a template to explore a range of factors that can control the formation of potential high entropy oxides. Using a combination of computational and experimental methods we interrogate the significance of various features including the tolerance factor, cation size variance, the decomposition rates of reactants, the stability of alternative products and the role of order on the lattice. By combining this information we are able to give insight to how these different features interplay and potential limit or aid the formation of high entropy oxides. We also comment on where the materials enter a high entropy, disordered state or where enthalpy plays a more significant role than might be expected. From our results, we offer thoughts on the differing rules governing the mixing processes in these perovskite phases.