In Situ High-Temperature Structural Analysis of High-Entropy Rare-Earth Sesquioxides

Abstract

High-entropy rare-earth (RE) sesquioxides (RE₂O₃) containing five cations in equimolar amounts have been investigated for a variety of applications, but little is known about their polymorphic behavior and coefficient of thermal expansion. Here, in this work, we evaluate the effect of the average ionic radius (AIR) on the polymorphism of high-entropy RE₂O₃. Powder samples of compositions 1 (Lu,Y,Ho,Nd,La)₂O₃ (AIR = 0.938 Å) and 2 (Gd,Eu,Sm,Nd,La)₂O₃ (AIR = 0.982 Å) were synthesized via a wet chemical method, and bead samples were prepared for aerodynamic levitation by melting the powders in a copper hearth. Structural transitions were monitored upon cooling from the melt to 1000 °C via in situ X-ray diffraction on aerodynamically levitated samples. The phase evolution was liquid, hexagonal H-type, and monoclinic B-type for composition 1 and liquid, cubic X-type, H-type, and B-type for composition 2. Based on their AIR, the general polymorphic transformations of the high-entropy RE₂O₃ follow the trend of single-RE RE₂O₃, but the transition temperatures differ from those of single-RE RE₂O₃. The coefficient of thermal expansion values of the B-type phase of compositions 1 and 2 are similar to those of Gd₂O₃ and previously published high-entropy RE₂O₃.