Optical grade (Gd_0.95-xLu_xEu_0.05)₃ Al₅O₁₂ ceramics with near-zero optical loss: Effects of Lu³⁺ doping on structural feature, microstructure evolution, and far-red luminescence

A chemical co-precipitation strategy was employed to synthesize a series of (Gd0.95-xLuxEu0.05)(3)Al5O12 (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 bulk specimens with x = 0.1, 0.3, 0.5, 0.7, and 0.95 nm were similar to 83.5%, 80.1%, 68.8%, 73.7%, and 82.2% at 710 nm (Eu3+ emission center), respectively, among which the sample of x = 0.1 exhibited the optical grade with near-zero optical loss in agreement with the defect-free single crystal (similar to 100% of the theoretical transmittance). The resulting particle and ceramic materials both presented characteristic Eu3+ emission arising from D-5(0)-> F-7(J) (J = 1-4) transition, where the dominant far-red emission at similar to 710 nm arising from D-5(0)-> F-7(4) transition overlapped with the absorption of phytochrome (P-FR). The photoluminescence excitation and photoluminescence intensities of (Gd0.95-xLuxEu0.05)(3)Al5O12 powders and ceramics generally increased at a higher Gd3+/Lu3+ ratio. Lu3+ dopants delayed the fluorescence lifetime while the bulk samples had shorter lifetime compared to the particle counterparts. The transparent (Gd0.85Lu0.1Eu0.05)(3)Al5O12 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 a current-dependent color rendering index. More importantly, our report marked the developmental stage of transparent ceramic materials towards zero optical loss.