A chemical co-precipitation strategy was employed to synthesize a series of (Gd_0.95-xLu_xEu_0.05)₃Al₅O₁₂ (x = 0.1−0.95) powder phosphors, followed by vacuum sintering to achieve transparent garnet ceramic phosphors. The density functional theory indicated Lu₃Al₅O₁₂ was formed in priority compared with Gd₃Al₅O₁₂ during solid-phase reaction. Upon high-temperature sintering, the Lu³⁺ substitution for Gd³⁺ suppressed point mass diffusion leading to a smaller grain size. The in-line transmittances of the bulk specimens with x = 0.1, 0.3, 0.5, 0.7, and 0.95 nm were ~83.5, 80.1, 68.8, 73.7, and 82.2% at 710 nm (Eu³⁺ emission center), respectively, among which the x = 0.1 sample exhibited optical grade with near-zero optical loss in agreement with the defect-free single crystal (~100% of the theoretical transmittance). The resulting particle and ceramic materials both presented characteristic Eu³⁺ emission arising from ⁵D₀→⁷F_J (J = 1−4) transition, where the dominant far-red emission at ~710 nm arising from ⁵D₀→⁷F₄ transition overlapped with the absorption of phytochrome P_FR. The photoluminescence excitation and photoluminescence intensities of (Gd_0.95-xLu_xEu_0.05)₃Al₅O₁₂ powders and ceramics generally increased at a higher Gd³⁺/Lu³⁺ ratio. Lu³⁺ dopants delayed the fluorescence lifetimes while the bulk samples had shorter lifetimes than the particle counterparts. The transparent (Gd_0.85Lu_0.1Eu_0.05)₃Al₅O₁₂ ceramic phosphor exhibited good thermal stability with a high thermal quenching temperature above 533 K. The designed ceramic phosphor converted light-emitting diode had a saturation injection current of 435 mA and current-dependent color rendering index. More importantly, our report marked the developmental stage of transparent ceramic materials towards zero optical loss.

A translucent Gd₂O₂S:Pr ceramic scintillator with an in-line transmittance of ~31% at 512 nm was successfully fabricated by argon-controlled sintering. The starting precipitation precursor was obtained by a chemical precipitation route at 80 °C using ammonia solution as the precipitate, followed by reduction at 1000 °C under flowing hydrogen to produce a sphere-like Gd₂O₂S:Pr powder with an average particle size of ~95 nm. The Gd₂O₂S:Pr phosphor particle exhibits the characteristic green emission from ³P_0,1→³H₄ transitions of Pr³⁺ at 512 nm upon UV excitation into a broad excitation band at 285–335 nm arising from 4f²→4f5d transition of Pr³⁺. Increasing Pr³⁺ concentrations induce two redshifts for the two band centers of 4f²→4f5d transition and lattice absorption on photoluminescence excitation spectra. The optimum concentration of Pr³⁺ is 0.5 at.%, and the luminescence quenching type is dominated by exchange interaction. The X-ray excited luminescence spectrum of the Gd₂O₂S:Pr ceramic is similar to the photoluminescence behavior of its particle. The phosphor powder and the ceramic scintillator have similar lifetimes of 2.93–2.99 μs, while the bulk material has rather higher external quantum efficiency (~37.8%) than the powder form (~27.2%).