Composition Engineering and Crystal Growth of High-Entropy Rare-Earth Monoclinic Aluminates

We generated high-entropy (HE) compositions of rare-earth (RE) monoclinic aluminates using a combinatorial approach, grew crystals from the melt using the micropulling down method, and investigated phase formation, microstructure, and elemental segregation. All RE elements, except for Pm and Tm, were used to generate RE4Al2O9 compounds of five RE elements taken in equimolar ratios. The distribution of HE formulations was demonstrated as a function of the average ionic radii (AIR). The compositions were grouped based on their AIR that is equal to those of RE ions in single-rare-earth compounds: Y4Al2O9 (Y group), Tb4Al2O9 (Tb group), Gd4Al2O9 (Gd group), and Nd4Al2O9 (Nd group). Single monoclinic phases and uniform microstructures were observed in the Y group. Dot-, line-, and core-like inclusions of the secondary REAlO3 perovskite phase were found in crystals of the Tb group and the Gd group in scanning electron micrographs. The concentration of the secondary phase increased with the fraction of bigger RE elements, such as La, Ce, Pr, Nd, and Sm in the HE formulation, as confirmed by X-ray diffraction. Incongruent melt solidification was found for compounds of the Nd group. Our results demonstrated that the stability of the monoclinic phase decreased with the AIR value in the sequence of the Y group, Tb group, Gd group, and Nd group, similar to their single-rare-earth compounds. Minor random variations in cell parameters between the seed and tail sides were found in crystals from the Y group, Tb group, and Gd group, indicating no pronounced elemental segregation. Based on the ionic radii of RE elements and their fraction in the HE formulation, recommendations were given on how to minimize the secondary phase. [GRAPHICS]