Abstract

High-entropy oxide (HEO) nanoceramics based on (Y0.2La0.2Gd0.2Eu0.2Er0.2)2O3 were synthesized at 600 °C for 10 min under pressures of 2, 4, 6, and 8 GPa using High-Pressure–Low-Temperature (HPLT) technology, and the resulting structural, phase, and optical changes were studied as a function of synthesis pressure. X-ray diffraction with Rietveld refinement showed that the initial single-phase cubic nanopowder decomposes under pressure into a mixture of cubic, monoclinic, and orthorhombic high-entropy phases: the cubic fraction falls from 67.3% at 2 GPa to 46.8% at 4 GPa, 42.9% at 6 GPa, and 31.1% at 8 GPa, while low-symmetry monoclinic and orthorhombic inclusions become correspondingly more abundant. Raman spectroscopy validated this evolution, with the 2 GPa sample showing a resolvable doublet near 364 and 353 cm−1 attributable to two coexisting cubic phases, while samples synthesized at 4–8 GPa converge on a single narrow band at 353 cm−1 that broadens with increasing pressure. Photoluminescence measurements revealed that the sample synthesized at 2 GPa exhibits the highest Eu3+ and Er3+ luminescence intensity, with emission and excitation intensities decreasing systematically as synthesis pressure increases. We attribute this decline to the growing fraction of low-symmetry monoclinic phase, whose C2h point-group sites impose parity-forbidden selection rules on the 5D0 → 7FJ transitions of Eu3+, combined with an increased probability of nonradiative relaxation at structural defects introduced by pressure. These results establish synthesis pressure as a practical lever for tuning the phase composition and luminescent efficiency of multi-lanthanide HEO nanoceramics and indicate that low pressures (~2 GPa) are preferable for optical applications requiring high luminescence intensity.

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