The challenge of growing rare-earth (RE) sesquioxide crystals can be overcome by tailoring their structural stability and melting point via composition engineering. This work contributes to the advancement of the field of crystal growth of high-entropy oxides. A compound with only small REs (Lu,Y,Ho,Yb,Er)2O3 maintains a cubic C-type structure upon cooling from the melt, as observed via in-situ high-temperature neutron diffraction on aerodynamically levitated samples. On the other hand, a compound with a mixture of small and large REs (Lu,Y,Ho,Nd,La)2O3 crystallizes as a mixture of a primary C-type phase with an unstable secondary phase. Crystals of compositions (Lu,Y,Ho,Nd,La)2O3 and (Lu,Y,Gd,Nd,La)2O3 were grown by the micro-pulling-down (mPD) method with a single monoclinic B-type phase, while a powder of (Lu,Y,Ho,Yb,Er)2O3 did not melt at the maximum operating temperature of an iridium-rhenium crucible. The minimization of the melting point of the two grown crystals is attributed to the mismatch in cation sizes. The electron probe microanalysis reveals that the general element segregation behavior in the crystals depends on the composition.

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.