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 Gd₃Al₅O₁₂ is stabilized by doping with smaller Lu³⁺, which then allows an effective incorporation of larger Eu³⁺ activators. The [(Gd_1?xLu_x)_1?yEu_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.     

Metal composition of Y2O3:Eu powder evaluated using particle analyzer

Y₂O₃:Eu red phosphor powder was synthesized with powders of metal–ethylenediaminetetraacetic acid (EDTA) complexes as a starting material. The compositional analysis of each metal–EDTA complexes particle and Y₂O₃:Eu particle was performed using a particle analyzer. In this study, first, the particles of a mixture of Y– and Eu–EDTA complexes were obtained by a spray drying method from solution consisting of Y– and Eu–EDTA·NH₄. Then, the metal–EDTA complex powder was fired in obtaining the Y₂O₃:Eu red phosphor. The metal composition of each particle was scattered for the powder of the metal–EDTA mixture, while it became narrow for the Y₂O₃:Eu powder. The intensity of cathodoluminescence obtained from the Y₂O₃:Eu powder increased with increasing the fired temperature. In addition, the maximum intensity was obtained from the sample with x=0.12 for Y_2−xO₃:Eu_x.     

Ferromagnetic semiconductors: Magnetic properties of the pyrochlores, (YxLu1−x)2V2O7 and (ScxLu1−x)2V2O7

Some magnetic properties of eight members of the pyrochlore systems (Y_xLu_1?x2V₂O₇ and (Sc_xLu_1?x)₂V₂O₇ are reported. Critical temperatures are determined for Lu₂V₂O₇ (72.5K) and the other phases to about ± 1K. Trends in T_c, π_c, C_m, μ_eff, and to a lesser extent, μ_SAT., occur which can be correlated, qualitatively, with thermogravometric data. From (Y_.40Lu_.60) to (Sc_.20Lu_.80) the quantities mentioned are nearly constant and analysis indicates the presence of V⁴⁺ only. C_m (0.47 cm³K/mole) and μ_eff (1.94 μB) are significantly greater than the spin-only values of 0.36 and 1.73 respectively. From (Sc_.30Lu_.70) to (Sc_.50Lu_.50) a reduced form of vanadium is detected, probably V³⁺, which increases with Sc content. This is accompanied by a sharp decline in T_c and π_c and a sharp increase in C_m and μ_eff.     

Shaped single crystal growth and scintillating application of Yb:(Gd,Lu)3(Ga,Al)5O12 solid solutions

Shaped single crystals of (Yb_0.05Lu_xGd_0.95−x)Ga₅O₁₂ (0.0x0.9) and Yb_0.15Gd_0.15Lu_2.7(Al_xGa_1−x)O₁₂ (0.0x1.0) were grown by the modified micro-pulling-down method. Continuous solid solutions with garnet structure and a linear compositional dependency of crystal lattice parameter in the system Yb:(Gd,Lu)₃(Ga,Al)₅O₁₂ are formed. Measured optical absorption spectra of the samples show 4f–4f transitions related to Gd³⁺ ion at 275 and 310 nm, and also an onset of charge transfer transitions from oxygen ligands to Gd³⁺ or Yb³⁺ cations below 240 nm. A complete absence of Yb³⁺ charge transfer luminescence under X-ray excitation in any of the investigated samples was explained by the overlapping of charge transfer absorption of Yb³⁺ by that of Gd³⁺ ions. For specific composition of Lu_1.5Gd_1.5Ga₅O₁₂ an intense defect––host lattice-related emission, which achieve of about 40% integrated intensity compared with Bi₄Ge₃O₁₂, was found.     

Sol-Gel Synthesis and Photoluminescence of RE<Subscript>x</Subscript>Lu<Subscript>2-<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> (RE = Y,Gd) Nanophosphors

RE_xLu_2-x O₃:Eu³⁺ (RE = Y, Gd) nanophosphors with x = 0.4, 1.0, and 1.6 were synthesized using rare-earth salicylate coordination polymers as precursors for the luminescent species RE_xLu_2-x O₃:Eu³⁺ (RE = Y, Gd) composed of polyethylene glycol both as a dispersing medium and fuel. Their particle sizes were around 100 nm as determined by scanning electron microscopy and x-ray diffraction.__________From Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 706–710.Original English Text Copyright © 2005 by Zhou, Yan.This article was submitted by the authors in English.     

The development of Ce3+-activated (Gd,Lu)3Al5O12 garnet solid solutions as efficient yellow-emitting phosphors

Abstract

Ce³⁺-activated Gd₃Al₅O₁₂ garnet, effectively stabilized by Lu³⁺ doping, has been developed for new yellow-emitting phosphors. The powder processing of [(Gd_1?xLu_x)_1?yCe_y]₃Al₅O₁₂ solid solutions was achieved through precursor synthesis via carbonate precipitation, followed by annealing. The resultant (Gd,Lu)AG:Ce³⁺ phosphor particles exhibit typical yellow emission at ～570 nm (5d–4f transition of Ce³⁺) upon blue-light excitation at ～457 nm (the ²F_5/2–5d transition of Ce³⁺). The quenching concentration of Ce³⁺ was determined to be ～1.0 at% (y = 0.01) and the quenching mechanism was suggested to be driven by exchange interactions. The best luminescent [(Gd_0.9Lu_0.1)_0.99Ce_0.01]AG phosphor is comparative to the well-known YAG:Ce³⁺ in emission intensity but has a substantially red-shifted emission band that is desired for warm-white lighting. The effects of processing temperature (1000–1500 °C) on the spectroscopic properties of the phosphors, especially those of Lu³⁺/Ce³⁺, were thoroughly investigated and discussed from the centroid position and crystal field splitting of the Ce³⁺ 5d energy levels.     

Synthesis and luminescent properties of BaLn2(1−x)ZnO5:2xTb3+ (Ln = Y, Gd) nanophosphors

The synthesis of BaLn_2(1?x)ZnO₅:2xTb³⁺ (Ln = Y, Gd) nanophosphors using solution combustion method with an aim to study the effect of sintering temperature and Tb³⁺ ions concentration on the luminescent properties has been investigated. Under UV excitation, BaY_2(1?x)ZnO₅ and BaGd_2(1?x)ZnO₅ nanoparticles exhibit apparent characteristic green emission from ⁵D₄ state to ⁷F_6?3 states of Tb³⁺ ions with the strongest at 544 nm. Luminescence concentration quenching could be observed when the Tb³⁺ ions contents were more than 4 mol% in both nanophosphors. The luminescence decay curves suggest monoexponential behavior of both nanophosphors. X-ray diffraction results confirmed the single-phased orthorhombic structure of both powders belonging to space group Pbnm. TEM analysis indicates the spherical morphology of nanoparticles with average grain size in the range of 85–95 nm. BaY_2(1?x)Tb_2x ZnO₅ and BaGd_2(1?x)Tb_2x ZnO₅ nanophosphors may have potential applications in field emission displays based on their uniform shape, low-cost synthetic route, and diverse luminescent properties.     

Crystal growth and characterization of Tm doped mixed rare-earth aluminum perovskite

In this work, we present results of structural characterization and optical properties including radio luminescence of (Lu_xGd_yY_0.99?x?yTm_0.01)AP single crystal scintillators for (x, y) = (0.30, 0.19), (0, 0.19) and (0, 0) grown by the micro-pulling-down (μ-PD) method. The grown crystals were single phase materials with perovskite structure (Pbnm) as confirmed by XRD and had a good crystallinity. The distribution of the crystal constituents in growth direction was evaluated, and significant segregation of Lu and Gd was detected in (Lu_0.30Gd_0.19Y_0.50Tm_0.01)AP sample. The crystals demonstrated 70% transmittance in visible wavelength range and some absorption bands due to Tm³⁺, Gd³⁺ and color centers were exhibited in 190–900 nm. The radioluminescence measurement under X-ray irradiation demonstrated several emission peaks ascribed to 4f–4f transitions of Tm³⁺ and Gd³⁺. The ratio of emission intensity in longer wavelength range was increased when Y was replaced by Lu or Gd.     

Luminescence of SrGdGa3O7:RE(RE = Eu, Tb) phosphors and energy transfer from Gd to RE

Eu³⁺- and Tb³⁺-activated SrGdGa₃O₇ phosphors were synthesized by the solid-state reaction and their luminescence properties were investigated. Sr(Gd_1 − xEu_x)Ga₃O₇ and Sr(Gd_1 − xTb_x)Ga₃O₇ formed continuous solid solution in the range of x = 0-1.0. Unactivated SrGdGa₃O₇ exhibited a typical characteristic excitation and emission of Gd ion. The SrGdGa₃O₇:xEu³⁺ and SrGdGa₃O₇:xTb³⁺ phosphors also showed the well-known Eu³⁺ and Tb³⁺ excitation and emission. The energy transfer from Gd³⁺ to Eu³⁺ and Tb³⁺ were verified by photoluminescence spectra. The dependence of photoluminescence intensity on Eu³⁺ and Tb³⁺ concentration were also studied in detail and the photoluminescence (PL) intensity of SrGdGa₃O₇:Eu and SrGdGa₃O₇:Tb were compared with commercial phosphors, Y₂O₃:Eu and LaPO₄:Ce,Tb. The luminescence decay measurements showed that the lifetimes of Eu³⁺ and Tb³⁺ were in the range of microsecond. The energy transfer from Gd³⁺ to Tb³⁺ was also observed in decay curve.     

Luminescent properties of YAG:Ce phosphor powders prepared by hydrothermal-homogeneous precipitation method

Y_3 − xCe_xAl₅O₁₂ (YAG:Ce³⁺) phosphor powders were successfully prepared by hydrothermal-homogenous precipitation (HHP) method, under mild conditions with inexpensive aluminum and yttrium nitrates as the starting materials and urea as homogenous precipitant. The pure YAG crystalline phase could be formed after hydrothermal treatment at 100 °C for 4 h and 240 °C for 20 h and postannealing process at 1200 °C for 2 h. All of the as-prepared YAG:Ce³⁺ powders did not have the CeO₂ phase. The photoluminescence spectrum of crystalline YAG:Ce³⁺ phosphors showed the emission intensity of phosphor increased with increasing the annealed temperature and reached its maximum as the molar fraction of cerium ion was 0.10, and also showed the maximum emission wavelength nearly unchanged with the calcination temperature and cerium doping concentration.     

Metal composition of Y2O3:Eu powder evaluated using particle analyzer

Y₂O₃:Eu red phosphor powder was synthesized with powders of metal–ethylenediaminetetraacetic acid (EDTA) complexes as a starting material. The compositional analysis of each metal–EDTA complexes particle and Y₂O₃:Eu particle was performed using a particle analyzer. In this study, first, the particles of a mixture of Y– and Eu–EDTA complexes were obtained by a spray drying method from solution consisting of Y– and Eu–EDTA$NH₄. Then, the metal–EDTA complex powder was fired in obtaining the Y₂O₃:Eu red phosphor. The metal composition of each particle was scattered for the powder of the metal–EDTA mixture, while it became narrow for the Y₂O₃:Eu powder. The intensity of cathodoluminescence obtained from the Y₂O₃:Eu powder increased with increasing the fired temperature. In addition, the maximum intensity was obtained from the sample with xZ0.12 for Y_2KxO₃:Eu_x.     

Luminescence and crystallinity of flame-made Y2O3:Eu3+ nanoparticles

Cubic and/or monoclinic Y₂O₃:Eu³⁺ nanoparticles (10–50 nm) were made continuously without post-processing by single-step, flame spray pyrolysis (FSP). These particles were characterized by X-ray diffraction, nitrogen adsorption and transmission electron microscopy. Photoluminescence (PL) emission and time-resolved PL intensity decay were measured from these powders. The influence of particle size on PL was examined by annealing (at 700–1300°C for 10 h) as-prepared, initially monoclinic Y₂O₃:Eu³⁺ nanoparticles resulting in larger 0.025–1 μm, cubic Y₂O₃:Eu³⁺. The influence of europium (Eu³⁺) content (1–10 wt%) on sintering dynamics as well as optical properties of the resulting powders was investigated. Longer high-temperature particle residence time during FSP resulted in cubic nanoparticles with lower maximum PL intensity than measured by commercial micron-sized bulk Y₂O₃:Eu³⁺ phosphor powder. After annealing as-prepared 5 wt% Eu-doped Y₂O₃ particles at 900, 1100 and 1300°C for 10 h, the PL intensity increased as particle size increased and finally (at 1300°C) showed similar PL intensity as that of commercially available, bulk Y₂O₃:Eu³⁺ (5 μm particle size). Eu doping stabilized the monoclinic Y₂O₃ and shifted the monoclinic to cubic transition towards higher temperatures.     

Synthesis of the Bi-2222 Compounds of Bi–Sr–Ln(Zr)–Cu–O (Ln = Sm,Eu, Gd,and Dy)

Bi-based superconducting compounds with the 2222 structure has been already synthesized in the Bi₂Sr₂- (Ln_1−x Ce _x )₂Cu₂O_10+y (Ln = Sm, Eu, and Gd) systems. One of the characteristics of these compounds is the existence of the fluorite-like (Ln_1−x Ce _x )₂O₂ block between two CuO₅ pyramids in the crystal structure. The tetravalent ions of Ce⁴⁺ are reported to be necessary to stabilize the 2222 structure. Recently, we have discovered that the Bi-2222 phase could be composed in the Bi₂Sr₂(Ln_2−x Zr _x )Cu₂O _z (Ln = Sm, Eu, Gd, and Dy) systems, where Zr⁴⁺ is used as a new tetravalent ion stabilizing the 2222 structure in stead of Ce⁴⁺. In the new system, nearly single 2222 phase samples have been obtained at the nominal composition of x=0.5 (Ln = Sm, Eu, and Gd) and in the range of 0.1≤x≤0.3 (Ln = Dy). Among them, the sample with Ln = Gd has the smallest resistivity at 273 K. But it is a semiconductor, and the conduction process at low temperatures is assumed to be followed by a two-dimensional VRH. The experimental results for the Gd samples with a partial substitution of Pb for Bi in the Bi₂Sr₂(Gd_2−x Zr _x )Cu₂O _z system are also reported.     

Morphology and luminescent properties of Al<Superscript>3+</Superscript>/Mg<Superscript>2+</Superscript>-doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> phosphor

Al³⁺/Mg²⁺ doped Y₂O₃:Eu phosphor was synthesized by the glycine-nitrate solution combustion method. In contrast to Y₂O₃:Eu which showed an irregular shape of agglomerated particles (the mean particle size >10 μm), the morphology of Al³⁺/Mg²⁺ doped Y₂O₃:Eu crystals was quite regular. Al³⁺/Mg²⁺ substituting Y³⁺ in Y₂O₃:Eu resulted in an obvious decrease of the particle size. Meanwhile, higher the Al³⁺/Mg²⁺ concentration, smaller the particle size. In particular, the introduction of Al³⁺ ion into Y₂O₃ lattice induced a remarkable increase of PL and CL intensity. While, for Mg²⁺ doped Y₂O₃:Eu samples, their PL and CL intensities decreased. The reason that causes the variation of PL and CL properties for Al³⁺ and Mg²⁺ doped Y₂O₃:Eu crystals was concluded to be related to sites of Al³⁺ and Mg²⁺ ions inclined to take and the difference of ion charge.     

Energy transfer and radiative recombination processes in (Gd, Lu)3Ga3Al2O12:Pr3+ scintillators

(Gd_xLu_3−x)Ga₃Al₂O₁₂:0.3 at.%Pr (x = 0.025, 0.05, 0.1, 0.2, 0.4, 0.6) (GLGAG:Pr) polycrystalline powders are prepared by solid-state reaction method. To better understand the luminescence mechanism, the optical diffuse reflectance, photoluminescence emission and excitation, X-ray excited luminescence spectra and decay kinetics of GLGAG:Pr were investigated in detailed, allowing the determination of energy transfer from 5d state of Pr³⁺ to 4f state of Gd³⁺, and the non-radiative relaxation from 5d to 4f state of Pr³⁺. Besides, the former process plays more negative role in the emission quenching of GLGAG:Pr than later one. Pr³⁺ ion is regarded as an ineffective activation ion in Gd-based multicomponent aluminate garnets. In addition, the wavelength-resolved thermoluminescence spectra of GLGAG:Pr were studied after UV and X-ray irradiation. It is revealed that the localized recombination processes from electron traps to lower lying 4f levels of Pr³⁺ occurs without populating the higher 5d levels of Pr³⁺.     

Structure and electrical conductivity of Ln2+x Hf2−x O7−x/2 (Ln = Sm-Tb; x = 0, 0.096)

Data are presented on the evolution of the pyrochlore structure in the Ln_2+x Hf_2?x O_7?δ (Ln = Sm, Eu; x = 0.096) solid solutions and Ln₂Hf₂O₇ (Ln = Gd, Tb) compounds prepared from mechanically activated oxide mixtures. Sm_2.096Hf_1.904O_6.952 is shown to undergo pyrochlore-disordered pyrochlore-pyrochlore (P-P1-P) phase transformations in the temperature range 1200–1670°C. The former transformation leads to a rise in 840°C conductivity from 10^?4 to 3 × 10^?3 S/cm in the samples synthesized at 1600°C, and the latter leads to a drop in 840°C conductivity to 6 × 10^?4 S/cm in the samples synthesized at 1670°C. The reduction in the conductivity of Sm_2.096Hf_1.904O_6.952 is accompanied by the disappearance of the assumed superstructure. In the range 1300–1670°C, Eu_2+x Hf_2?x O_7?δ (x = 0.096) and Ln₂Hf₂O₇ (Ln = Gd, Tb) have a disordered pyrochlore structure. The highest 840°C conductivity is offered by Eu_2.096Hf_1.904O_6.952, Gd₂Hf₂O₇, and Tb₂Hf₂O₇ synthesized at 1670°C: 7.5 × 10^?3, 5 × 10^?3, and 2.5 × 10^?2 S/cm, respectively.     

Synthesis and morphological control of rare earth oxide nanoparticles by solvothermal reaction

Eu doped Y₂O₃ and some kinds of other rare earth oxides nanoparticles such as Er₂O₃, Nd₂O₃, Ho₂O₃, Lu₂O₃, and Dy₂O₃ were prepared by a simple co-precipitation-solvothermal treatment-calcination process, where the co-precipitated amorphous hydroxide precursors obtained by adding rare earth nitrate solutions in ammonia solutions were heated in solvents such as water, alcohols and glycols, followed by calcination in air. The morphology of rare earth oxide particles strongly depended on the solvothermal reaction medium but not related to the kind of rare earth oxide. The powders prepared in water and ethanol possessed nanowire structure, where the aspect ratio of powder treated in water was higher than that in ethanol. The powders prepared by co-precipition-solvothermal treatment-calcination process using ethylene glycol consisted of near-spherical nanoparticles whereas that prepared by conventional co-precipitation-calcination method consisted of hardly agglomerated submicron particles. The nanoparticles of Eu³⁺ doped Y₂O₃ prepared by co-precipition-solvothermal treatment-calcination process showed similar intensity of photoluminescence with the submicron particles by co-precipition-calcination process.     

A red phosphor for nUV LED based on (Y,Gd)BO3:Eu

(Y,Gd)BO₃:Eu³⁺ based phosphors were prepared by combustion technique followed by solid state sintering in air. The 4f-4f excitation at 394 nm of (Y_0.54Gd_0.46)_x (BO₃)_y:Eu³⁺ exhibits red emission at 593 nm. Its photoluminescence (PL) efficiency depends critically on the BO₃ to (Y,Gd) molar ratio. Optimal luminescence was obtained at the y/x molar ratio of 1.38. A 12-fold increase in its luminescence efficiency was seen with an increase in Eu concentration from 0.5 to 8.4 mol% at this y/x ratio. At 8.4 mol% of Eu concentration, the PL sensitivity of (Y_0.54Gd_0.46)_x (BO₃)_y:Eu³⁺ is 40% higher than that of the commercial (Y,Gd)BO₃:Eu³⁺ phosphor. In view of the high luminescence efficiency, this phosphor could serve as a potential candidate for application as a red phosphor for nUV LED apart from its known application in plasma display panels.     

A red-emitting heavy doped phosphor Li6Y(BO3)3:Eu for white light-emitting diodes

A series of Eu³⁺ activated Li₆Y_1−xEu_x(BO₃)₃ (0.05 ? x ? 1) phosphors were synthesized by solid-state reaction method. The structures and photoluminescent properties of the phosphors were investigated at room temperature. The results of XRD patterns indicate that these phosphors are isotypic to the monoclinic Li₆Gd(BO₃)₃. The excitation spectra indicate that these phosphors can be effectively excited by near UV (370-410 nm) light. The red emission from transition ⁵D₀→⁷F₂ is dominant. The emission spectra exhibit strong red performance (CIE chromaticity coordinates: x = 0.65, y = 0.35), which is due to the ⁵D₀−⁷F_J transitions of Eu³⁺ ions. The relationship between the structure and the photoluminescent properties of the phosphors was studied. The concentration quenching occurs at x ≈ 0.85 under near UV excitation. Li₆Y(BO₃)₃:Eu³⁺ has potential application as a phosphor for white light-emitting diodes.     

Effect of Y3+ codoping on luminescence decays of the 4I13/2 level in Er3+-doped Al2O3 powders prepared by a sol-gel method

The luminescence lifetime of the 0.01 mol.%-0.1 mol.% Er³⁺- and 0–20 mol.% Y³⁺-codoped Al₂O₃ powders prepared at a sintering temperature of 900°C in a non-aqueous sol-gel method has been investigated to explore the enhanced mechanism of photoluminescence properties of the Er³⁺-doped Al₂O₃ by Y³⁺ codoping. For the 0.1 mol.% Er³⁺-Y³⁺-codoped Al₂O₃ powders, the measured lifetime of Er³⁺ gradually increases with increasing Y³⁺ concentration. Consequently, codoping with 20 mol.% Y³⁺ leads to an increase in the measured lifetime from 3.5 to 5.8 ms. By comparing the measured lifetime for different Er³⁺ concentrations in the Al₂O₃ powders, the radiative lifetime of both the Er³⁺-doped and the Er³⁺-Y³⁺-codoped Al₂O₃ powders is estimated to be about 7.5 ms. Infrared absorption spectra indicate that Y³⁺ codoping does not change the-OH content in the Er³⁺-Y³⁺-codoped Al₂O₃ powders. The prolonged luminescence lifetime of the ⁴I_13/2 level of Er³⁺ in Er³⁺-doped Al₂O₃ powders by Y³⁺ codoping is ascribed to the decrease in the energy transfer rate between the Er³⁺ ions and the Er³⁺ and -OH, respectively, due to the suppressed interaction between Er³⁺ ions.