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

Abstract

The metastable garnet lattice of Gd 3 Al 5 O 12 is stabilized by doping with smaller Lu 3+ , which then allows an effective incorporation of larger Eu 3+ activators. The [(Gd 1−x Lu x ) 1−y Eu y ] 3 Al 5 O 12 (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 5 D 0 → 7 F 1 magnetic dipole transition of Eu 3+ ) 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 Eu 3+ was estimated at ∼5 at.% (y = 0.05), and the quenching was attributed to exchange interactions. Partial replacement of Gd 3+ with Lu 3+ up to 50 at.% (x = 0.5) while keeping Eu 3+ at the optimal content of 5 at.% does not significantly alter the peak positions of the CTB and 5 D 0 → 7 F 1 emission bands but slightly weakens both bands owing to the higher electronegativity of Lu 3+ . 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 0.7 Lu 0.3 ) 0.95 Eu 0.05 ] 3 Al 5 O 12 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.

Recent progress in advanced optical materials based on gadolinium aluminate garnet (Gd3Al5O12)

“…The minimum amount of Lu 3+ (∼17 at%) calculated from the ionic size of Tb 3+ (0.1040 nm for CN = 8), however, is significantly larger than the ∼10 at% found in practice (figure 3(b)). This indicates that stable garnet solid solutions exist if the average ionic size of (Ln 1 ,Ln 2 ) 3+ pair lies in between those of Gd 3+ (0.1053 nm for CN = 8) and Tb 3+ , in agreement with the fact that TbAG [28–30] and even (Gd 0.9 Lu 0.1 )AG [31, 32] can be further doped with larger Eu 3+ (0.1066 nm, CN = 8) and Ce 3+ (0.1143 nm, CN = 8) for luminescence. Taking the average ionic size of (Gd 0.9 Lu 0.1 ) 3+ (~0.1045 nm) as a standard, Li et al [33] analyzed the minimum amounts of various small Ln 3+ that are needed for GAG stabilization, and the x value was predicted to be ∼0.5 for Tb 3+ , 0.3 for Dy 3+ (0.1027 nm), 0.22 for Y 3+ (0.1019 nm), 0.2 for Ho 3+ (0.1015 nm), 0.15 for Er 3+ (0.1004 nm), 0.13 for Tm 3+ (0.0994 nm), and 0.11 for Yb 3+ (0.0985 nm).…”

“…For this, the emission of YAG:Eu and LuAG:Eu is dominated by the parity-law allowed 5 D 0 → 7 F 1 magnetic dipole transition at ∼590 nm rather than the forced 5 D 0 → 7 F 2 electric dipole transition at ∼610 nm as observed from the well-known Y 2 O 3 :Eu red phosphor. A [(Gd 1− x Lu x ) 1− y Eu y ]AG solid solution has recently been developed as efficient red phosphor with Lu 3+ as the lattice stabilizer, and the effects of various factors on optical properties were thoroughly investigated [31]. Taking [(Gd 0.7 Lu 0.3 ) 1− y Eu y ]AG for example, the material was shown to be efficiently excitable with the charge transfer band (CTB) at ∼239 nm to produce a sharp orange–red emission at 591 nm (figure 7), with CIE chromaticity coordinates of (0.62, 0.38) and a full width at half maximum of only ∼6 nm for the emission peak.…”

“…The optimal Eu 3+ content was experimentally determined to be ∼5 at% ( y = 0.05), and the quenching mainly resulted from exchange interactions, possibly via phonon assisted three Eu 3+ ion nonresonant interactions. Greatly improved emission intensity and quantum yield, shortened fluorescence lifetime, and increased asymmetry factor of luminescence (the I 591 / I 610 intensity ratio) were observed along with increasing synthesis temperature up to 1500 °C [31], primarily owing to lattice perfection, defect elimination, and particle growth. The [(Gd 0.7 Lu 0.3 ) 0.95 Eu 0.05 ]AG phosphor synthesized at 1500 °C has internal/external quantum efficiencies (%) of 83.2/56.1, an asymmetry factor of ∼2.85, and a fluorescence lifetime of ∼4.1 ms for the 591 nm emission [31].…”

“…Greatly improved emission intensity and quantum yield, shortened fluorescence lifetime, and increased asymmetry factor of luminescence (the I 591 / I 610 intensity ratio) were observed along with increasing synthesis temperature up to 1500 °C [31], primarily owing to lattice perfection, defect elimination, and particle growth. The [(Gd 0.7 Lu 0.3 ) 0.95 Eu 0.05 ]AG phosphor synthesized at 1500 °C has internal/external quantum efficiencies (%) of 83.2/56.1, an asymmetry factor of ∼2.85, and a fluorescence lifetime of ∼4.1 ms for the 591 nm emission [31]. The lifetime is close to that (4.66 ms) reported for YAG:Eu [35] but is significantly longer than those (generally 0.5–2.5 ms) of the well-known red phosphors of Y 2 O 3 :Eu [36, 37], (Gd 1− x Ln x ) 2 O 3 :Eu (Ln = Y, Lu) [22, 38], and La 2 O 2 S:Eu [39], since the Eu 3+ activator takes the highly symmetric D 2 lattice site in garnet.…”

“…The lifetime is close to that (4.66 ms) reported for YAG:Eu [35] but is significantly longer than those (generally 0.5–2.5 ms) of the well-known red phosphors of Y 2 O 3 :Eu [36, 37], (Gd 1− x Ln x ) 2 O 3 :Eu (Ln = Y, Lu) [22, 38], and La 2 O 2 S:Eu [39], since the Eu 3+ activator takes the highly symmetric D 2 lattice site in garnet. Increasing Lu incorporation up to x = 0.5 would lower excitation/emission and also blue-shift the CTB due to gradually decreased covalency of the host lattice ( χ = 1.27 for Lu 3+ ), for which a minimized Lu content was recommended as long as the garnet lattice can be effectively stabilized [31]. Similar phenomena were observed in the development of (Gd 1− x Ln x ) 2 O 3 :Eu red phosphors (Ln = Y, Lu) [38].…”