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, 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.

Crystal growth and phase formation of high‐entropy rare‐earth monoclinic aluminates

“…Measurements up to temperatures close to the melting point of these compounds would be advisable to capture the full dependence of the structural behavior on temperature. Maximum experimental temperatures around 2100°C would be required since the melting points of single‐RE RE 4 Al 2 O 9 are in the range of 1950–2040°C, 4 and the melting points of high‐entropy oxides is usually lower or similar to their single‐cation analogous oxides 24,41,42 . Such extreme temperature measurements require careful treatment to avoid reactions between the sample and sample holder.…”

“…One strategy to perform these measurements is to use a containerless sample environment. Aerodynamic levitation and laser heating have been previously used to perform high‐temperature neutron and XRD on high‐entropy oxides 24,41,43–47 …”

“…Maximum experimental temperatures around 2100 • C would be required since the melting points of single-RE RE 4 Al 2 O 9 are in the range of 1950-2040 • C, 4 and the melting points of high-entropy oxides is usually lower or similar to their single-cation analogous oxides. 24,41,42 Such extreme temperature measurements require careful treatment to avoid reactions between the sample and sample holder. One strategy to perform these measurements is to use a containerless sample environment.…”

“…Aerodynamic levitation and laser heating have been previously used to perform high-temperature neutron and XRD on high-entropy oxides. 24,41,[43][44][45][46][47]…”

Thermal Expansion and Enthalpies of Phase Transformation and Fusion of Er2O3 and Tm2O3 from Experiment and Computation

“…They were denoted by Goldschmidt as B-type ( C 2/ m ) and A-type ( P m 1), respectively. The high temperature hexagonal (H, P 6 3 / mmc ) and cubic (X, ImP m ) phases cannot be quenched in binary or multicomponent , RE 2 O 3 (however, the reflections assigned to X-phase were identified in Tm 2 O 3 and Lu 2 O 3 after irradiation with swift heavy ions (185 MeV Xe)) . Thermodynamics of phase transitions involving these phases can only be studied experimentally in situ, which is challenging since they occur above 2000 °C.…”