Influence of Mg<Superscript>2+</Superscript>/Ga<Superscript>3+</Superscript> doping on luminescence of Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce<Superscript>3+</Superscript> phosphors

Mg²⁺/Ga³⁺ doped Y₃Al₅O₁₂:Ce³⁺ phosphors were synthesized through a solid state reaction. The phase and luminescent of the synthesized phosphors were investigated. For Ga³⁺ codoped Y_2.96Ce_0.04Al_(5?x)Ga_xO₁₂ phosphors, the emission intensity increases with the increase of Ga³⁺ concentration up to Y_2.96Ce_0.04Al_4.80Ga_0.20O₁₂ and then decreases with a further increase of Ga³⁺ concentration, but the emission peak shifts to shorter wavelength continuously in the Ga³⁺ doping concentration range of 0.05–0.25. For Mg²⁺/Ga³⁺ codoped Y_2.96Ce_0.04Al_(4.8?y)Ga_0.20Mg_yO₁₂ phosphors, the emission intensity decreases and the emission peak shifts to longer wavelength continuously in the Mg²⁺ doping concentration range of 0.02–0.12. The emission spectra of Y_2.96Ce_0.04Al_(4.8?y)Ga_0.20Mg_yO₁₂ phosphors demonstrate that the codoped Mg²⁺/Ga³⁺ ions not only induce the enhancement of Y_2.96Ce_0.04Al₅O₁₂ emission intensity but also lead to the red shift of Y_2.96Ce_0.04Al₅O₁₂ emission peak. The decay lifetimes decrease in Mg²⁺/Ga³⁺ codoped Y_2.96Ce_0.04Al₅O₁₂ phosphors due to defects formed by substitutions of Y³⁺ by Mg²⁺/Ga³⁺.     

Wavelength dependence of Verdet constant of Pr doped terbium gallium garnet crystal

Samples of the magneto-active material – Pr³⁺: Tb₃Ga₅O₁₂ crystals with Pr³⁺ ion concentration of 1%, 2%, 3% and 5%. The Verdet constant measurement has been carried out at room temperature in the 400–1500 nm range for all crystal samples and was compared with a pure Tb₃Ga₅O₁₂ material. A high value of the Verdet constant for 5% Pr³⁺: Tb₃Ga₅O₁₂ crystal was obtained at room temperature – namely, 324.5, 200.1 and 68.7 rad/(T·m) for 532, 632.8 and 1064 nm, respectively. The Verdet constant of Pr doped TGG crystal at 1064 nm is much more higher than that of TGG. The superior performance of the materials indicates that Pr³⁺: Tb₃Ga₅O₁₂ crystals have great potential to meet the increasing demand for magneto-optical devices in the VIS-NIR wavelength.     

Preparation and luminescence properties of Y<Subscript>3−y</Subscript>Al<Subscript>5−x</Subscript>Ga<Subscript>x</Subscript>O<Subscript>12</Subscript>:Ce<Superscript>3+</Superscript><Subscript>y</Subscript> phosphors

Y_3?yAl_5?xGa_xO₁₂:Ce³⁺_y phosphors were prepared by high temperature solid state reaction method. The crystal structures, the influence of Ga³⁺ concentration on the photoluminescence (PL), cathodoluminescence, thermal stability and morphology of the phosphors were studied in detail. The results indicated that diffraction angle of the samples decreased gradually with the increase of Ga³⁺ ions content in XRD pattern. The emission peak of the spectra show a progressive blue-shift, the intensity increased first and then decreased and the optimal Ga³⁺ concentration in Y_2.94Al_5?xGa_xO₁₂:Ce³⁺_0.06 phosphors is x?=?0.75. The critical concentration of Ce³⁺ in YAGG:Ce³⁺ phosphors is affected with the ratio of Ga³⁺ to Al³⁺ and the Y_2.9Al_4.25Ga_0.75O₁₂:Ce³⁺_0.1 phosphor showed the best performance on PL. However, the optimum concentration of Y_3?yAl_5?xGa_xO₁₂:Ce³⁺_y phosphors is x?=?1.5 and y?=?0.04 when they were excited by cathode ray.     

Single crystals growth and magneto-thermal properties of Dy3Ga5O12 garnet

Single crystals of Dy₃Ga₅O₁₂ garnet have been grown by the Czochralski method and the anisotropy of magnetic entropy evaluated for use as a magnetic material for a magnetic refrigeration in place of Gd₃Ga₅O₁₂ single crystals. As the size of the single crystal is large enough, and the anistropy is small, this crystal is suited to use for magnetic refrigeration in the temperature range of 1.8 to 15 K.     

X-ray diffraction study of La3Ga5.5Ta0.5O14 and La3Ga5.5Nb0.5O14 langasite-type single crystals

The structure of langatate (as-grown and vacuum-annealed: LGT-I and LGT-II, respectively) and langanite (seed- and tail-end portions: LGN-III and LGN-IV, respectively) single crystals grown by the Czochralski technique from charges of nominal composition La₃Ga_5.5M_0.5O₁₄ = La₃(Ga_0.5M_0.5)(1)Ga₃(2)Ga₂(3)O₁₄ (M=Ta⁵⁺, Nb⁵⁺) is studied by x-ray diffraction. The LGT-I and LGT-II crystals are shown to differ in the Ga and Ta distributions over crystallographic sites: La₃(Ga_0.52)Ta_0.48(2) ⁵⁺Ga₃(Ga_0.94Ta_0.06(1) ³⁺)₂O₁₄ in LGT-I (Ta⁵⁺ in the octahedral site Ga(1) and Ta³⁺ in the trigonal-pyramidal site Ga(3)) and La₃(Ga_0.55Ta_0.45(2) ⁵⁺)Ga₃Ga₂O_13.93(2)0.07 in LGT-II (Ta⁵⁺ in the octahedral site Ga(1) and oxygen vacancies in O(1)). The increased Ta content is responsible for the lower structural perfection of LGT-I. LGN-III and LGN-IV have essentially identical compositions, La₃(Ga_0.47Nb_0.53(1) ⁵⁺)Ga₅O₁₄ and La₃(Ga_0.48Nb_0.52(1) ⁵⁺)Ga₅O₁₄, respectively, but differ in polarity.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 485–492.Original Russian Text Copyright © 2005 by Kuzmicheva, Tyunina, Domoroshchina, Rybakov, Dubovskii.     

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.     

Growth and characterization of Yb doped garnet crystals for scintillator application

Several concentration of Yb-doped Lu₃Al₅O₁₂ (Yb:LuAG) and Lu₃Ga₅O₁₂ (Yb:LGG) single crystals were grown by the micro-pulling-down method. The crystals were seeded-grown in the 1 1 1 direction and transparent and crack free crystals were obtained. Photoluminescence spectra and decay kinetics of these crystals were studied. Charge transfer luminescence of Yb³⁺ was observed in both crystals. Mean decay time of about 25 ns at 90 K and strong thermal quenching at room temperature was measured for Yb 5%:LuAG. Radioluminescence intensity was compared to the standard BGO sample.     

Optical and scintillation properties of Nd-doped complex garnet

Nd 1% doped complex garnet scintillators were prepared by Furukawa and their optical and scintillation properties were investigated on a comparison with previously reported Nd-doped YAG. Chemical compositions of newly developed complex garnets were Lu₂Y₁Al₅O₁₂, Lu₂Y₁Ga₃Al₂O₁₂, Lu₂Gd₁Al₅O₁₂, Lu₂Gd₁Ga₃Al₂O₁₂, Gd₁Y₂Al₅O₁₂, Gd₁Y₂Ga₃Al₂O₁₂, and Gd₃Ga₃Al₂O₁₂. They all showed 50–80% transmittance from ultraviolet to near infrared wavelengths with several absorption bands due to Gd³⁺ or Nd³⁺ 4f–4f transition. In X-ray induced radioluminescence spectra, all samples exhibited intense lines at 310 nm due to Gd³⁺ or 400 nm due to Nd³⁺ depending on their chemical composition. Among them, the highest scintillation light yield was achieved by Lu₂Y₁Al₅O₁₂. Typical scintillation decay times of them resulted 1.5–3 μs. Thermally stimulated glow curve after 1 Gy exposure and X-ray induced afterglow were also investigated.     

Effects of gallia additions on sintering behavior of gadolinia-doped ceria

The effects of gallia additions on the sintering behavior of gadolinia-doped ceria were systematically investigated from the following aspects: the variation in sintered density, the variation in grain size, and the existing forms of Ga₂O₃ in CeO₂.Sintered density increased with increasing Ga₂O₃ content up to 5 mol.% and then it decreased with further addition of Ga₂O₃. Grain size also increased with increasing Ga₂O₃ content up to 5 mol.% and then decreased with further addition of Ga₂O₃. Decrease in grain size was caused by a pinning effect of Ga₂O₃ precipitation at grain boundaries. Lattice constant decreased with increasing Ga₂O₃ content up to 5 mol.%. This decrease will be due to the substitution of smaller Ga³⁺ ions for Ce⁴⁺ ions in the CeO₂ structure. According to the results obtained from scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses, the solubility limit of Ga₂O₃ in Ce_0.8Gd_0.2O_1.9 ceramics can be estimated to be nearly 5 mol.%. The addition of Ga₂O₃ up to the solubility limit was found to promote the sintering properties of Gd₂O₃-doped CeO₂.     

Magnetic properties of (Dy1−xGdx)3Ga5O12 garnet single crystals

Magnetic properties of Dy_1–x Gd _x )₃Ga₅O₁₂ (DGGG) garnet single crystals were calculated using the Weiss molecular field theory and also measured using the vibrating sample magnetometer in the temperature range 4.2–40 K in the effective magnetic field from 0–7 T. The magnetic properties of DGGG single crystals are distributed between those of Dy₃Ga₅O₁₂ (DGG) and Gd₃Ga₅O₁₂ (GGG) single crystals, but are considerably closer to those of DGG. Based on the magnetic properties, the magnetic entropy change, S_M, was evaluated in the temperature range below 15 K. DGGG single crystals are a prospective material for magnetic refrigeration below 15 K.     

The refractive indices of some garnet crystals at 1.15 μm

Refractive index measurements for Y₃Fe₅O₁₂, (LaY)₃Ga_.33Fe_4.67O₁₂ and Ga₃Gd₅O₁₂ were obtained at 1.152 μm by observing optical waveguiding in epitaxial thin films.     

Thermal expansion of some garnets

Lattice constants of the garnets {Gd₃} [Gd_0.03Ga_1.97] (Ga₃)O₁₂, {Y₃} [Y_0.505Ga_1,495] (Ga₃)O₁₂, Y₃Fe₄GaO₁₂, {Y₃} [In_0.6Ga_1.4] (Ga₃)O₁₂, {Ca₂La} [Zr₂] (Ga₃O₁₂, {Y_1.05Ca_1.95}Ti_1.95Ga_3.05O₁₂, Ca₃Sn₃Ga₂O₁₂, {Ca_2.5Zr_2.5} [Zr₂] (Ga₃)O₁₂, {Na₂Ca} [Zr₂] (Ge₃)O₁₂ have been measured in the temperature range 296–1400°K. The most important results are: 1) Crystals in the system {Y₃} [Y_xGa_2−x] (Ga₃)O₁₂ are potentially good substrates for epitaxial films of the system Y₃Fe_5−xGa_xO₁₂ and 2) Our Czochralski grown crystals of GdGaG actually have the formula {Gd₃} [Gd_0.03Ga_1.97]-(Ga₃)O₁₂; the difference in the thermal expansions of the stoichiometric and Czochralski-grown materials is not significant.     

Composition engineering of single crystalline films based on the multicomponent garnet compounds

The paper demonstrates our last achievement in development of the novel scintillating screens based on single crystalline films (SCF) of Ce doped multicomponent garnets using the Liquid Phase Epitaxy (LPE) method. We report in this work the optimized content and excellent scintillation properties of SCF of Lu_3-xGd_xAl_5-yGa_yO₁₂, Lu_3-xTb_xAl_5-yGa_yO₁₂ and Tb_xGd_xAl_5-yGa_yO₁₂ garnet compounds grown by the LPE method from PbOB₂O₃ based melt-solution onto Gd₃Al_2.5Ga_2.5O₁₂ and YAG substrates.We also show that the Tb_1.5Gd_1.5Al_2.5Ga_2.5O₁₂:Ce SCF possess the highest light yield (LY) in comparison with all ever grown garnet SCF scintillators. Namely, the LY of these SCF exceeds by 3.8 and 1.85 times the LY values of the best samples of YAG:Ce and LuAG:Ce SCF scintillators, respectively. The SCF samples of the mentioned compounds show low thermoluminescence in the above room temperature range and relatively fast scintillation decay time t_1/e in the 180–200 ns range.     

Growth and Faraday rotation characteristics of Tb3−xNdxGa5O12 single crystal

Tb_3−xNd_xGa₅O₁₂ single crystal with dimension of Φ22 mm × 28 mm and a good optical quality was grown by the Czochralski method. X-ray powder diffraction was carried out and lattice parameters were calculated, which showed that the single crystal belongs to cubic crystal system. The transmission spectrum in the wavelength range of 450–1500 nm, which indicated the crystal has low absorption coefficient at 900–1450 nm. The Verdet constants of Tb_3−xNd_xGa₅O₁₂ at 532, 633 and 1064 nm wavelengths calculated by the extinction method are 225, 145 and 41 radm⁻¹ T⁻¹, respectively, which are larger than that of commercial TGG (Tb₃Ga₅O₁₂) reported. The magneto-optical figures of merit of the crystal calculated is 3162°/dB at 1064 nm, and the extinction ratio is larger than that of commercial TGG.     

Time-resolved spectroscopy of intrinsic luminescence of Y3Ga5O12 and (LaLu)3Lu2Ga3O12 single crystals

The nature of intrinsic luminescence of Y₃Ga₅O₁₂ (YGG) and (LaLu)₃Lu₂Ga₃O₁₂ (LLGG) single crystals grown from a melt was determined. In the case of a YGG single crystal containing Y_Ga antisite defects with a concentration of 0.25–0.275 at.% the intrinsic luminescence was considered as a superposition of luminescence of self-trapped excitons (STE), luminescence of excitons localized near antisite defects (LE(AD) centers) and luminescence caused by a recombination of an electron with a hole captured at Y_Ga antisite defects. Due to a large (2–3%) concentration of Lu_La antisite defects in LLGG single crystals the intrinsic luminescence was a superposition mainly of the LE(AD) center emission and the recombination luminescence of Lu_La antisite defects. The energy structure of the mentioned centers in YGG and LGGG hosts was determined from the excitation spectra of their luminescence under excitation by synchrotron radiation in the range of the fundamental absorption edge of these garnets.     

Time-resolved spectroscopy of intrinsic luminescence of Y3Ga5O12 and (LaLu)3Lu2Ga3O12 single crystals

The nature of intrinsic luminescence of Y₃Ga₅O₁₂ (YGG) and (LaLu)₃Lu₂Ga₃O₁₂ (LLGG) single crystals grown from a melt was determined. In the case of a YGG single crystal containing Y_Ga antisite defects with a concentration of 0.25–0.275 at.% the intrinsic luminescence was considered as a superposition of luminescence of self-trapped excitons (STE), luminescence of excitons localized near antisite defects (LE(AD) centers) and luminescence caused by a recombination of an electron with a hole captured at Y_Ga antisite defects. Due to a large (2–3%) concentration of Lu_La antisite defects in LLGG single crystals the intrinsic luminescence was a superposition mainly of the LE(AD) center emission and the recombination luminescence of Lu_La antisite defects. The energy structure of the mentioned centers in YGG and LGGG hosts was determined from the excitation spectra of their luminescence under excitation by synchrotron radiation in the range of the fundamental absorption edge of these garnets.     

Crystal growth and characterization of Yb-doped Gd3Ga5O12

Single crystals of (Yb_xGd_1−x)₃Ga₅O₁₂ (0.0 ≤ x ≤ 1.0) have been grown by the micro-pulling-down method. Formation of continuous solid solutions with a garnet structure was confirmed. Composition dependence of the lattice constant, thermal diffusivity, specific heat capacity and thermal conductivity was investigated. Assignment of the Yb³⁺-energy levels in Gd₃Ga₅O₁₂-host lattice has been performed by using absorption, emission and Raman spectroscopy measurements at both, room temperature and at 12 K.     

Experimental investigation of the quantum amplification effect for magnetostatic waves in ferrite-paramagnet structures

The effect of quantum amplification of the forward bulk magnetostatic waves (MSWs) in a composite structure of yttrium iron garnet Y₃Fe₅O₁₂ (YIG) and Al₂O₃:Cr³⁺ (ruby) was studied. The maximum amplification coefficient at 4.2 and 1.6 K was 12 and 25 dB, respectively. No MSW amplification was observed in an epitaxial structure comprising an YIG film grown on a gadolinium gallium garnet Gd₃Ga₅O₁₂ (GGG) substrate, which is explained by a strong paramagnetic absorption of MSWs in the GGG substrate crystal.     

Crystal growth of oxide and fluoride materials for optical, piezoelectric and other applications

High quality oxide and fluoride single crystals for optical, piezoelectric and other applications have been grown by advanced crystal growth techniques. La₃Nb_0.5Ga_5.5O₁₄ and La₃Ta_0.5Ga_5.5O₁₄ piezoelectric single crystals of size and quality comparable to La₃Ga₅SiO₁₄ (langasite), have been produced. The piezoelectric and device properties of the crystals were investigated. A search for new langasite-type materials was also performed. Growth conditions of (La,Sr)(Al,Ta)O₃ and Ca₈La₂(PO₄)₆O₂ single crystals are discussed. These crystals have excellent lattice matching with GaN, a promising material for optoelectronic devices. Promising optical micro-crystals—K₃Li₂Nb₅O₁₅, KNbO₃ and Y₃Al₅O₁₂—and new structural materials, Al₂O₃Y₃Al₅O₁₂ eutectic fibers, have been grown by the micro-pulling-down (-PD) technique. The advantages of the -PD technique have been shown. Ce-doped fluoride single crystals—LiCaAlF₆, LiYF₄ and BaLiF₃—have been grown for solid state UV laser applications. Growth results and optical characterizations are discussed.     

Electron traps in Gd<Subscript>3</Subscript>Ga<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>12</Subscript>:Ce garnets doped with rare-earth ions

The curves of thermally stimulated luminescence of Gd₃Ga₃Al₂O₁₂:Ce³⁺ ceramics (a nominally pure sample and samples doped with rare-earth ions) are measured in the temperature range of 80–550 K. The depth and the frequency factor of electron traps established by Eu and Yb impurities are determined. An energy-level diagram of rare-earth ions in the bandgap of Gd₃Ga₃Al₂O₁₂ is presented.