Effect of processing route on the crystal structure and physical properties of bixbyite high-entropy oxides

•High-entropy rare earth oxides (HE-REOs) with the form (Y-La-Ce-Nd-Gd)2O3+β were synthesized.•Both mechanochemical (ball milling) and chemical (sol-gel) methods were employed.•Compositional uniformity of as-synthesized powders were in the following order: sol-gel>wet milling>dry milling.•The synthesized HE-REOs exhibited very high phase stability. [Display omitted] (Y-La-Ce-Nd-Gd)2O3+β high-entropy rare earth oxides (HE-REOs) were synthesized using both mechanochemical (ball milling) and chemical (sol-gel) methods. Energy-dispersive X-ray spectroscopy revealed that the compositional uniformity of the as-synthesized powders was the highest in the sol-gel process, followed by that in wet milling and dry milling. The sol-gel process was advantageous because it yielded oxide powders with the best compositional uniformity and required a lower annealing temperature to obtain a single phase compared to the ball-milled powders. However, following heat treatment at 1600 °C, similar chemical uniformity and physical properties of the oxides were obtained regardless of the synthesis method. The formation of single-phase Ia3̅ structures was confirmed by X-ray diffraction and Rietveld refinement analyses. A detailed transmission electron microscopy analysis revealed that a Fm3̅m structure formed at low temperatures. The thermodynamic parameters of high-entropy alloys, high-entropy transition metal oxides, and HE-REOs were calculated and compared. This analysis confirmed that the HE-REOs synthesized in the present study had very high phase stability.