Highly dense translucent CeO2-δ·(RE, Y, Sm, La)2O3 (RE = Dy, Gd) high-entropy ceramics for multi-wavelength emission

This study reports the synthesis of translucent, high-entropy oxide (HEO) ceramics with the composition CeO2-δ·(RE, La, Sm, Y)2O3 (RE = Dy, Gd), and a single-phase bixbyite structure (space group Ia-3). The materials were prepared by reactive sintering at 1600°C in air, achieving translucency through optimized ball milling of precursor oxides and refinement of processing parameters. X-ray diffraction and scanning electron microscopy confirmed phase purity and a highly dense microstructure, with relative densities exceeding 99%. The samples were translucent in the visible and near-infrared part of the spectrum. To improve the transparency, hot isostatic pressing (HIP) at 1600°C and 185 MPa was employed on the samples sintered for 6 h. HIP induced partial phase separation, impairing translucency. The effect of dwell time on luminescence properties of reactive sintered samples was also studied. Ultraviolet‒visible spectroscopy revealed a narrower bandgap and an enhanced photoluminescence (PL) intensity in the sample sintered for 6 h, as a result of a higher concentration of oxygen vacancies. PL under 302 nm excitation displayed multi-wavelength emissions peaking at 432, 572, and 653 nm, producing near-cold white light. Time-resolved PL decay analysis indicated multiple luminescence centers with efficient energy transfer (e.g., Ce3+ to Sm3+/Dy3+/Gd3+).