Fabrication of (Dy0.2Ho0.2Er0.2Tm0.2Yb0.2)2O3​ high-entropy oxide via microwave-assisted synthesis and spark plasma sintering: Comparative microstructural characterization

Abstract

In this study, a novel high-entropy oxide (HEO) with the nominal composition (Dy₀.₂Ho₀.₂Er₀.₂Tm₀.₂Yb₀.₂)₂O₃ was successfully synthesized via two distinct routes: microwave sintering (MIC) and spark plasma sintering (SPS). The primary objective was to conduct a comparative investigation into the influence of each fabrication method on the phase evolution, microstructural characteristics, and chemical homogeneity of the resulting compound. X-ray diffraction (XRD) analysis, complemented by Rietveld refinement, confirmed the formation of a single-phase cubic bixbyite structure with the Ia-3 space group for both samples, devoid of any secondary phases. The lattice parameters were calculated to be 10.57 Å for the SPS sample and 10.54 Å for the microwave-sintered sample. Williamson-Hall analysis revealed that the SPS process yielded a significantly finer crystallite size (47.8 nm) and higher micro-strain ((1.8 ± 0.5) $\times {10}^{-3}$) compared to the microwave method (92.4 nm and (4 ± 1.5) $\times {10}^{-4}$, respectively), attributable to the rapid heating kinetics of SPS that inhibit grain growth. Microstructural evaluation via scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) demonstrated dense, well-sintered microstructures with uniform, equiaxed grains. Elemental mapping using energy-dispersive X-ray spectroscopy (EDS) confirmed the homogeneous distribution of all five rare-earth elements at both the micro- and nano-scales, validating the entropy-stabilized nature of the single phase. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis verified the trivalent oxidation state (RE³⁺) for all cations and a stable oxygen environment within the lattice, with no significant chemical state alterations induced by the different sintering routes. The results establish a clear relationship between the synthesis pathway and the microstructural evolution, demonstrating that both methods produce pure HEOs.