Composition Engineering\nand Crystal Growth of High-Entropy\nRare-Earth Monoclinic Aluminates

We generated high-entropy (HE) compositions\nof rare-earth\n(RE)\nmonoclinic aluminates using a combinatorial approach, grew crystals\nfrom the melt using the micropulling down method, and investigated\nphase formation, microstructure, and elemental segregation. All RE\nelements, except for <i>Pm</i> and Tm, were used to generate\nRE<sub>4</sub>Al<sub>2</sub>O<sub>9</sub> compounds of five RE elements\ntaken in equimolar ratios. The distribution of HE formulations was\ndemonstrated as a function of the average ionic radii (AIR). The compositions\nwere grouped based on their AIR that is equal to those of RE ions\nin single-rare-earth compounds: Y<sub>4</sub>Al<sub>2</sub>O<sub>9</sub> (Y group), Tb<sub>4</sub>Al<sub>2</sub>O<sub>9</sub> (Tb group),\nGd<sub>4</sub>Al<sub>2</sub>O<sub>9</sub> (Gd group), and Nd<sub>4</sub>Al<sub>2</sub>O<sub>9</sub> (Nd group). Single monoclinic phases\nand uniform microstructures were observed in the Y group. Dot-, line-,\nand core-like inclusions of the secondary REAlO<sub>3</sub> perovskite\nphase were found in crystals of the Tb group and the Gd group in scanning\nelectron micrographs. The concentration of the secondary phase increased\nwith the fraction of bigger RE elements, such as La, Ce, Pr, Nd, and\nSm in the HE formulation, as confirmed by X-ray diffraction. Incongruent\nmelt solidification was found for compounds of the Nd group. Our results\ndemonstrated that the stability of the monoclinic phase decreased\nwith the AIR value in the sequence of the Y group, Tb group, Gd group,\nand Nd group, similar to their single-rare-earth compounds. Minor\nrandom variations in cell parameters between the seed and tail sides\nwere found in crystals from the Y group, Tb group, and Gd group, indicating\nno pronounced elemental segregation. Based on the ionic radii of RE\nelements and their fraction in the HE formulation, recommendations\nwere given on how to minimize the secondary phase.