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

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, 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 angstrom) and 2 (Gd,Eu,Sm,Nd,La)2O3 (AIR = 0.982 angstrom) were synthesized via a wet chemical method, and bead samples were prepared for aerody-namic levitation by melting the powders in a copper hearth. Structural transitions were monitored upon cooling from the melt to 1000 degrees 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.

Automated summary

This study investigates the polymorphic behavior and coefficient of thermal expansion of high-entropy rare-earth sesquioxides (RE2O3) and reveals the influence of average ionic radius (AIR) on their polymorphism. Powder and bead samples of two different compositions were synthesized and analyzed using in situ X-ray diffraction. The polymorphic transitions of high-entropy RE2O3 follow a similar trend to single-RE RE2O3, but with different transition temperatures.