Effects of Gd Substitution on Sintering and Optical Properties of Highly Transparent (Y_0.95-xGd_xEu_0.05)₂O₃ Ceramics

Highly transparent (Y_0.95-xGd_xEu_0.05)₂O₃ (x = 0.15-0.55) ceramics have been fabricated by vacuum sintering at the relatively low temperature of 1700°C for 4 h with the in-line transmittances of 73.6%-79.5% at the Eu³⁺ emission wavelength of 613 nm (~91.9%-99.3% of the theoretical transmittance of Y_1.34Gd_0.6Eu_0.06O₃ single crystal), whereas the x = 0.65 ceramic undergoes a phase transformation at 1650°C and has a transparency of 53.4% at the lower sintering temperature of 1625°C. The effects of Gd³⁺ substitution for Y³⁺ on the particle characteristics, sintering kinetics, and optical performances of the materials were systematically studied. The results show that (1) calcining the layered rare-earth hydroxide precursors of the ternary Y-Gd-Eu system yielded rounded oxide particles with greatly reduced hard agglomeration and the particle/crystallite size slightly decreases along with increasing Gd³⁺ incorporation; (2) in the temperature range 1100°C-1480°C, the sintering kinetics of (Y_0.95-xGd_xEu_0.05)₂O₃ is mainly controlled by grain-boundary diffusion with similar activation energies of ~230 kJ/mol; (3) Gd³⁺ addition promotes grain growth and densification in the temperature range 1100°C-1400°C; (4) the bandgap energies of the (Y_0.95-xGd_xEu_0.05)₂O₃ ceramics generally decrease with increasing x; however, they are much lower than those of the oxide powders; (5) both the oxide powders and the transparent ceramics exhibit the typical red emission of Eu³⁺ at ~613 nm (the ⁵D₀→⁷F₂ transition) under charge transfer (CT) excitation. Gd³⁺ incorporation enhances the photoluminescence and shortens the fluorescence lifetime of Eu³⁺.