Empirical prediction of the crystal structure of high-entropy zirconates based on cationic geometrical parameters.

High-entropy oxides (HEOs) are oxides with at least five different cations randomly sharing the same sublattice position and are gaining increasing attention from the scientific community due to their interesting properties. However, given their compositional complexity, a fundamental study on how their chemical composition influences their crystalline structure and stability is needed. In this work, HEOs of the general formula A2Zr2O7 (where A = combination of 5 or more rare earth ions) have been investigated. The ionic size difference and the ionic radius ratio of the A-site cations have been correlated to predict the crystalline phase of the material: either defect fluorite or pyrochlore. For the phase-pure samples, the cationic parameters were also correlated with the lattice parameter, structural strain, and oxygen displacement, to evidence predictable trends. In addition, the behavior of cerium ions and the applicability of a tolerance factor similar to that for perovskites were studied.