In Situ High-Temperature Structural Analysis of High-Entropy\nRare-Earth Sesquioxides

Abstract

High-entropy\nrare-earth (RE) sesquioxides (RE₂O₃) containing\nfive cations in equimolar amounts have been investigated\nfor a variety of applications, but little is known about their polymorphic\nbehavior and coefficient of thermal expansion. Here, we evaluate the\neffect of the average ionic radius (AIR) on the polymorphism of high-entropy\nRE₂O₃. 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\nwet chemical method, and bead samples were prepared for aerodynamic\nlevitation by melting the powders in a copper hearth. Structural transitions\nwere monitored upon cooling from the melt to 1000 °C via in situ\nX-ray diffraction on aerodynamically levitated samples. The phase\nevolution was liquid, hexagonal H-type, and monoclinic B-type for\ncomposition 1 and liquid, cubic X-type, H-type, and B-type for composition\n2. Based on their AIR, the general polymorphic transformations of\nthe high-entropy RE₂O₃ follow the trend of single-RE\nRE₂O₃, but the transition temperatures differ\nfrom those of single-RE RE₂O₃. The coefficient\nof thermal expansion values of the B-type phase of compositions 1\nand 2 are similar to those of Gd₂O₃ and previously\npublished high-entropy RE₂O₃.