Formation of alkoxy-derived Y3Al5O12

Effective lattice stabilization of gadolinium aluminate garnet (GdAG) via Lu3+ doping and development of highly efficient (Gd,Lu)AG:Eu3+ red phosphors

The metastable garnet lattice of Gd₃Al₅O₁₂ is stabilized by doping with smaller Lu³⁺, which then allows an effective incorporation of larger Eu³⁺ activators. The [(Gd_1-x Lu _x )_1-y Eu _y ]₃Al₅O₁₂ (x = 0.1-0.5, y = 0.01-0.09) garnet solid solutions, calcined from their precursors synthesized via carbonate coprecipitation, exhibit strong luminescence at 591 nm (the ⁵D₀ → ⁷F₁ magnetic dipole transition of Eu³⁺) upon UV excitation into the charge transfer band (CTB) at ∼239 nm, with CIE chromaticity coordinates of x = 0.620 and y = 0.380 (orange-red). The quenching concentration of Eu³⁺ was estimated at ∼5 at.% (y = 0.05), and the quenching was attributed to exchange interactions. Partial replacement of Gd³⁺ with Lu³⁺ up to 50 at.% (x = 0.5) while keeping Eu³⁺ at the optimal content of 5 at.% does not significantly alter the peak positions of the CTB and ⁵D₀ → ⁷F₁ emission bands but slightly weakens both bands owing to the higher electronegativity of Lu³⁺. The effects of processing temperature (1000-1500 °C) and Lu/Eu contents on the intensity, quantum efficiency, lifetime and asymmetry factor of luminescence were thoroughly investigated. The [(Gd_0.7Lu_0.3)_0.95Eu_0.05]₃Al₅O₁₂ phosphor processed at 1500 °C exhibits a high internal quantum efficiency of ∼83.2% under 239 nm excitation, which, in combination with the high theoretical density, favors its use as a new type of photoluminescent and scintillation material.

Particle Size Effects in YAG:CR Phosphors

Chromium doped yttrium aluminum garnet (Y3Al5O12:Cr or YAG:Cr) phosphors were synthesized by hydrolysis of aqueous nitrate solutions. The resulting as-synthesized powder was amorphous and crystallized into the YAG structure at ~1000°C. The light output as a function of annealing temperature was found to increase with increasing temperature. Particle size, adsorbed surface species or residual impurities, lattice stability and chromium site occupation were investigated as possible explanations. It was found that there was a small decrease in lattice parameter upon annealing, the particle size increased roughly five times, and residual or adsorbed surface species were present for the low temperature annealed powders. There was no evidence that kinetic barriers to diffusion of Cr ions into the luminescent sites occurred.