Crystal Structure Stabilization Of Gadolinium Aluminate Garnet (Gd₃Al₅O₁₂) And Development Of New Phosphors

Pioneer investigations have been done on lattice stabilization of Gd3Al5O12(GdAG) garnet and based on which the development of new phosphors. Through precursor synthesis via carbonate precipitation (coprecipitation and urea-based homogenous precipitation) followed by proper calcination, high performance phosphors of (Gd,Lu)AG:RE3+(RE=Eu, Dy, Ce, Tb, Yb/Tm, Yb/Er, and Yb/Ho) have been developed. Utilizing the non-radiative energy transfer from the Gd3+ions in the host lattice, significantly enhanced photoluminescence has been achieved for various types of activators. The effects of synthesis temperature, particle morphology (size/shape), Lu/RE contents, crystal structure, coordination symmetry, and energy transfer on various aspects of luminescence of the resultant phosphors have been systematically studied in depth, and the following main conclusions have been reached:(1) Doping GdAG with significantly smaller Lu3+to form (Gd,Lu)AG (LnAG, Ln=Gd and Lu) solid solutions effectively stabilizes the garnet lattice against thermal decomposition. Precursors for (Gd1-xLux)AG (x=0-0.5), with a general formula of (NH4)xLn3Al5(OH)y(CO3)z·nH2O, were synthesized via carbonate coprecipitation from mixed nitrate solutions with ammonium hydrogen carbonate as the precipitant. The results showed that 10 at% of Lu3+(x=0.1) has been able to stabilize GdAG and that Lu3+doping effectively lowers the temperature of garnet crystallization. The precursors of x≥0.3 convert to pure LnAG at a low temperature of ～1000℃ via the intermediates of Ln4Al2O9 (LnAM) and LnAlO3 (LnAP). The carbonate precursors are loosely agglomerated and the resultant LnAG powders show good dispersion and fairly uniform particle morphologies. The (Gd,Lu)AG solid solutions exhibit decreasing lattice parameters and increasing optical bandgaps along with more Lu3+incorporation. Base on these observation, effectively stabilizing the garnet lattice of GdAG has also been achieved by doping with other small rare-earth ions (Tb3+-Yb+ and Y3+);(2) [(Gd1-xLux)1-yEuy]AG (x=0.1-0.5,y=0.01-0.09) garnet solid-solutions, calcined from their precursors synthesized via carbonate coprecipitation, exhibit strong orange red emissions at 591 nm·(the 5Do→7F1 magnetic dipole transitions of Eu3+) upon UV excitation into the charge transfer band (CTB) at～239 nm, with CIE chromaticity coordinates of (0.62,0.38). The quenching concentration of Eu3+ was found to be ～5 at%(y=0.05), and the quenching mechanism was determined to be exchange interactions. Partially replacing Gd3+ with Lu3+ up to 50 at%(x=0.5) while keeping Eu3+at the optimal content of 5 at% does not alter appreciably peak positions of the CTB and 5Do→7F1 emission bands but tends to weaken the both owing to the higher electronegativity of Lu3+. The effects of processing temperature (1000-1500 ℃) and Lu/Eu contents on the intensity, quantum efficiency, lifetime, and asymmetry factor of luminescence were thoroughly investigated. The [(Gdo.7Luo.3)o.95Euo.o5]AG phosphor processed at 1500℃ exhibits a high internal quantum efficiency of ～83.2% under 239nm excitation, which, in combination with its high theoretical density, may allow it to be used as a new type of photoluminescent and scintillation material;(3) Eu3+ doped Gd4Al209 (GdAM), GdA103 (GdAP), and GdAG (containing 10 at% of Lu3+ for lattice stabilization) have been developed in this work as efficient red-emitting phosphors. With coprecipitated carbonate precursors, phase evolution studies found minimum processing temperatures of ～1000,1100, and 1300℃ for the three phosphors to crystallize as pure phases, respectively. Compared with their yttrium aluminate counterparts, the gadolinium-based phosphors exhibit red-shifted O2--Eu3+ charge transfer excitation band (CTB) centers due to the lower electronegativity of Gd3+ and appreciably higher quantum yields of photoluminescence owing to the occurrence of efficient Gd3+→Eu3+energy transfer. The Eu3+ in GdAM, GdAP and (Gd0.9Lu0.1)AG have C1,Cs and D2 site symmetries, repectively, and thus emit vivid red, red, and orange-red colors. The optimal Eu3+ contents were determined to be ～7.5 at% for GdAM and 5.0 at% for both GdAP and (Gd,Lu)AG, and concentration quenching of luminescence was suggested to be due to exchange interactions. Fluorescence decay analysis found a shorter lifetime for the phosphor powder processed at a higher temperature or with a higher Eu3+content. At the same calcining temperature of 1300 ℃, the lifetime of (Gdo.95Euo.o5)AM, (Gd0.95Eu0.05)AP and [(Gd0.9Lu0.1)0.95Eu0.05]AG was found to be 2.42±0.01 ms (611 nm),1.78±0.01 ms (617 nm) and 5.30±0.06 ms (591 nm), respectively;(4) RE3+-doped and Lu3+-stabilized gadolinium aluminate garnet solid solutions of [(Gd1-xLux)1-yREy]AG(x=0.1-1.0 andy=0-0.10, RE=Tb, Ce, and Dy) have been developed as efficient green (Tb3+), yellow (Ce3+) and white (Dy3+) phosphors via calcining their respective precursors prepared by carbonate coprecipitation. The effects of processing temperature and Lu/RE contents on phase evolution, crystal structure, particle morphology, PLE/PL properties, and fluorescence lifetime of the phosphors have been thoroughly investigated. The quenching concentrations of Tb3+, Ce3+ and Dy3+ were determined to be ～10 at%,1.0 at% and 2.5 at%, and the quenching mechanism was suggested to be exchange interactions (for Tb3+ and Ce3+) and dipole-dipole interactions (for Dy3+). Compared with the RE3+ doped YAG and LuAG phosphors, the advantages of gadolinium-based phosphors developed in the paper have the following advantages, besides their high theoretical densities:The [(Gd0.9Lu0.1)0.9Tb0.1]AG phosphor processed at 1500℃ exhibits high internal and external quantum efficiencies of ～98.9% and 78.5% under 276 nm excitation, respectively; The [(Gd0.9Lu0.1)0.99Ce0.01]AG phosphor is comparative to the well-known YAG:Ce3+ phosphor in emission intensity but has an appreciably red-shifted emission band that is desired for warm-white lighting. The observed luminescence properties are the results of combined contributions from the centroid position and crystal field splitting of the Ce3+ 5d energy levels; Owing to the efficient Gd3+→Dy3- energy transfer, the (Gd0.8Lu0.2)0.975Dy0.025]AG phosphor exhibits simultaneously strong blue and yellow emissions, and the intensity of the 483 run emission (λex=275 nm. the 8S7/2→6Ij transition of Gd3+. indirectly exciting Dy3+) is ～6 and 3 times of those of the (Lu0.975Dy0.025)AG and (Yo.975Dyo.o25)AG phosphors (λex=352 nm. the 6H15/2→4I11/2+4M15/2+6P7/2 transition of Dy3+, directly exciting Dy3+), respectively. (Gd,Lu)AG:Dy3+ phosphor has CIE color coordinates of (0.33,0.35), and thus emits ideal white light;(5) Two doubly-doped phosphor systems of [(Gd0.8Lu0.2)0.9-x-Tb0.1Eux]AG and [(Gd0.8Lu0.2)0.99-x Ce0.01Tb,]AG, with good dispersion and uniform particle morphology, have been converted at 1300℃ from their respective precursors synthesized by via carbonate coprecipitation. For the [(Gd0.8Lu0.2)0.9-x Tb0.1Eux]AG system, successively weaker Tb3 emission at 545 nm was observed owing to the Tb3+→Eu3+ energy transfer. The Eu3+ emission at 592 nm steadily gains intensity owing to the Gd3+→Eu3+ and Tb3+→Eu3+ energy transfers up to x=0.03 and then deteriorates owing to concentration quenching. At x=0.03, the efficiency of energy transfer was found to be as high as ～83.2% and the transfer mechanism was suggested to be dipole-quadrupole electric interactions. The emission color can be tuned from approximately green to yellowish-orange by varying the Tb/Eu ratio. Fluorescence lifetime was found to decrease with increating Eu3+ content for both the 545 and 592 nm emissions. For the [(Gd0.8Lu0.2)0.99-xCe0.01Tbx]AG system, significantly improved Ce3+ emission was achieved with the Gd3+→Ce3+ and Tb3+→Ce3+ energy, transfers. Transfer mechanism was suggested to be electric multipole interactions;(6) The urea-based homogeneous precipitation technique was employed in the preparation of basic carbonate precursors for well dispersed [(GdxLu1-x)0.95Eu0.05]AG phosphor spheres. The effects of synthesis parameters were systematically investigated, and size control was achieved for the uniform phosphor spheres. The Al(NO3)3 to NH4Al(SO4)2 molar ration was found to have decisive effects and a ratio of about 1.0 yield uniform spheres. Particle size was found to decrease with increasing urea concentration, and larger particles were confirmed to exhibit a better luminescence and a shorter lifetime. Though Gd3+ content (x=0.1-0.4) does not alter appreciably particle morphology, peak position of the 5D0→7F1 emission band or lifetime, but tends to improve Eu3+ emission owing to the Gd3+→Eu3+energy transfer;(7) Three new types of up-conversion phosphors of (Gd,Lu)AG:Yb3+/RE3+(RE=Tm, Er, and Ho), with good particle dispersion, have been prepared by calcing their carbonate precursors. Lu3+ doping not only stabilizes the garnet lattice but also increases the theoretical density of material. Under laser excitation at 978 nm, the (Gd,Lu)AG:Yb3+/RE3+ phosphors display vivid blue (for Tm3+) and green emissions (for Er3+ and Ho3+). The up-conversion luminescence can be well explained with three-photon (for Tm3+) and two-photon (for Er3+ and Ho3+) processes. Replacing Gd3+ with Lu3+ up to 50 at%(x=0.5) does not alter appreciably peak positions of the up-conversion emission bands but tends to lower the emission intensity in each case, while raising the calcination temperature can significantly improve the up-conversion luminescence.