Optical grade (Gd 0.95- x Lu x Eu 0.05) 3Al 5O 12 ceramics with near-zero optical loss: Effects of Lu 3+ doping on structural feature, microstructure evolution, and far-red luminescence

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.