Crystal structure and thermal expansion behaviour of high-entropy perovskite (Y0.2Nd0.2Sm0.2Eu0.2Er0.2)AlO3

Two series of single-phase five-component aluminate perovskites with the composition (Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2 )AlO 3 were obtained for the first time using the sol-gel citrate method and the solution combustion technique. In both these cases, single-phase perovskite materials with average crystallite sizes of 38 and 76 nm, respectively, are formed after heat treatment of the synthesis products in air at 1000 °C. These materials remain stable after additional annealing for 4 h at 1100 °C; however, further heat treatment at 1200 °C leads to the partial decomposition of both materials with the precipitation of a garnet phase, the relative fraction of which gradually increases with rising annealing temperature up to 1500 °C. Complete sets of structural parameters for the high-entropy perovskite (Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2 )AlO 3 were obtained for the first time based on experimental X-ray powder diffraction data using full-profile Rietveld refinement. The evolution of the (Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2 )AlO 3 perovskite structure in the temperature range from RT to 773 K was studied using in situ temperature-dependent X-ray powder diffraction. It was found that this HE perovskite shows strongly anisotropic thermal expansion with relative elongation along the [010] direction ca. 2 times weaker compared with [100] and [001] directions. Comprehensive crystal chemical analysis of five-component HE perovskite (Y 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Er 0.2 )AlO 3 in comparison with the reference data for the single RE compounds REAlO 3 revealed that its crystal structure follows well the empirical relations earlier established for the rare-earth aluminate series, indicating that the structure of HE perovskite materials is mainly governed by the geometrical factor.