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

This review article summarizes the recent achievements in stabilization of the metastable lattice of gadolinium aluminate garnet (Gd₃Al₅O₁₂, GAG) and the related developments of advanced optical materials, including down-conversion phosphors, up-conversion phosphors, transparent ceramics, and single crystals. Whenever possible, the materials are compared with their better known YAG and LuAG counterparts to demonstrate the merits of the GAG host. It is shown that novel emission features and significantly improved luminescence can be attained for a number of phosphor systems with the more covalent GAG lattice and the efficient energy transfer from Gd³⁺ to the activator. Ce³⁺ doped GAG-based single crystals and transparent ceramics are also shown to simultaneously possess the advantages of high theoretical density, fast scintillation decay, and high light yields, and hold great potential as scintillators for a wide range of applications. The unresolved issues are also pointed out.     

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

Abstract

This review article summarizes the recent achievements in stabilization of the metastable lattice of gadolinium aluminate garnet (Gd₃Al₅O₁₂, GAG) and the related developments of advanced optical materials, including down-conversion phosphors, up-conversion phosphors, transparent ceramics, and single crystals. Whenever possible, the materials are compared with their better known YAG and LuAG counterparts to demonstrate the merits of the GAG host. It is shown that novel emission features and significantly improved luminescence can be attained for a number of phosphor systems with the more covalent GAG lattice and the efficient energy transfer from Gd³⁺ to the activator. Ce³⁺ doped GAG-based single crystals and transparent ceramics are also shown to simultaneously possess the advantages of high theoretical density, fast scintillation decay, and high light yields, and hold great potential as scintillators for a wide range of applications. The unresolved issues are also pointed out.     

X-ray study of gadolinium gallium garnet epitaxial layers containing divalent Co ions

Structural perfection of gadolinium gallium garnet (GGG; Gd₃Ga₅O₁₂) epitaxial layers with incorporated divalent Co ions has been studied by means of high resolution X-ray diffraction, X-ray topography, transmission electron microscopy and optical spectroscopy. Epitaxial layers were grown by liquid phase epitaxy from super-cooled high temperature solution with different concentration of Co₃O₄ and GeO₂ on both sides of the polished < 111> oriented GGG substrates. In order to facilitate incorporation of Co²⁺ ions into the garnet lattice optically inert Ge⁴⁺ ions have to be introduced first. High structural perfection is a prerequisite to obtain absorption spectra required for the use GGG epitaxial layers as tunable infrared absorbers.     

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.     

Dislocations in neodymium gallium garnet (Nd3Ga5O12 or NdGG)

Dislocations in imperfect (1 1 1) and (1 0 0) wafers of neodymium gallium garnet (Nd₃ Ga₅ O₁₂ or NdGG) have been examined using etch pits and the bi-refringence method. Long straight dislocations, large prismatic loops, small prismatic loops, dislocation nodes, and low-angle grain boundaries were observed. The Burgers vectors of some of the dislocations lay along the 〈1 1 0〉 or 〈1 1 1〉 directions. Dislocations with Burgers vectors along 〈1 0 0〉 may have been present, but conclusive evidence for this was not obtained. The diameter of the smallest prismatic loop was 3.1μm. It is thought to be the smallest dislocation loop observed by the bi-refringence method. The rotation across the low-angle grain boundaries was ～10^?6°.     

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.     

Optical Spectra of Gd3Ga5O12:Mn Crystals

The absorption and luminescence spectra of gadolinium gallium garnet crystals activated with Mn during high-temperature solution growth are analyzed in relation to the valence state of Mn. The results indicate that, in contrast to melt growth, the flux growth process stabilizes manganese in the valence state 3+.     

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.     

Free carrier absorption in self-activated PbWO4 and Ce-doped Y3(Al0.25Ga0.75)3O12 and Gd3Al2Ga3O12 garnet scintillators

Nonequilibrium carrier dynamics in the scintillators prospective for fast timing in high energy physics and medical imaging applications was studied. The time-resolved free carrier absorption investigation was carried out to study the dynamics of nonequilibrium carriers in wide-band-gap scintillation materials: self-activated led tungstate (PbWO₄, PWO) ant two garnet crystals, GAGG:Ce and YAGG:Ce. It was shown that free electrons appear in the conduction band of PWO and YAGG:Ce crystals within a sub-picosecond time scale, while the free holes in GAGG:Ce appear due to delocalization from Gd³⁺ ground states to the valence band within a few picoseconds after short-pulse excitation. The influence of Gd ions on the nonequilibrium carrier dynamics is discussed on the base of comparison the results of the free carrier absorption in GAGG:Ce containing gadolinium and in YAGG without Gd in the host lattice.     

(Y3-x-yREx)Al5O12:Cey(RE=Tb,Gd)荧光粉晶体结构与光致发光

采用化学共沉淀法合成具有Tb、Gd稀土掺杂的Y3Al5O12:Ce3 荧光粉,通过XRD和荧光光谱研究其晶体结构和光致发光.结果表明,(Y2.95-xTbx)Al5O12:Ce3 和(Y2.95-xGdx)Al5O12:Ce3 系列荧光粉具有与Y3Al5O12:Ce3 相同的石榴石晶体结构,但晶胞参数随Tb、Gd取代Y而略有增加;随Tb、Gd掺杂量增加,荧光粉发射波长逐渐红移,当x=2.95时,发射波长λmax分别从532nm红移至551和542nm;根据晶体场理论解释了荧光粉发射波长红移现象.这种发射波长红移的荧光粉能够显著地增强蓝光LED芯片与荧光粉组合形成的白光中红光成分,进而改善白光LED光源质量.     

Kapitza conductance and thermal conductivity of copper niobium and aluminium in the range from 1.3 to 2.1 K

The Kapitza conductance and thermal conductivity of ofhc-copper, niobium, ultra high purity aluminium, and of the aluminium alloy 6061 A1 have been measured in the temperature range from 1.3 to 2.1 K, yielding both quantities in the same steady state experiment. The temperature dependence of the Kapitza conductance, h_o, was between T^3.3 and T^4.6 for the different samples, which is higher than the most frequently observed T³ dependence. The magnitude of h_o for both ofhc-copper and aluminium agrees well at 1.9 K with an empirical prediction, but for niobium it is a factor of two to four lower than the value predicted. At 1.9 K, h_o is higher by a factor of two for an annealed and chemically polished niobium sample than for an untreated sample. The thermal conductivity measured from ofhc-copper and 6061 A1 as in good agreement with the value calculated from the resistivity of these materials and the Wiedemann-Franz law. The measured thermal conductivity obtained for an annealed niobium sample is a factor of 2.8 higher than the highest published value.     

Temperature and pressure dependence of the optical properties of Cr3+-doped Gd3Ga5O12 nanoparticles

Since the crystal-field strength at the Cr(3+) site is very close to the excited-state crossover (ESCO), this work investigates the optical properties of Cr(3+)-doped Gd(3)Ga(5)O(12) (GGG) nanoparticles as a function of temperature and pressure in order to establish the effect of the ESCO on the optical behaviour of nanocrystalline GGG. Luminescence, time-resolved emission and lifetime measurements have been performed on GGG:0.5% Cr(3+) nanoparticles in the 25-300 K temperature range, as well as under hydrostatic pressure up to 20 GPa. We show how low temperature and high pressure progressively transforms Cr(3+)(4)T(2) --> (4)A(2) broadband emission into a ruby-like (2)E --> (4)A(2) luminescence. This behaviour together with the lifetime dependence on pressure and temperature are explained on the basis of the spin-orbit interaction between the (4)T(2) and (2)E states of Cr(3+).     

Effect of microstructural evolution from nano to micron grain size regime towards structural,magnetic, electrical and microwave properties of gadolinium iron garnet (Gd3Fe5O12)

Journal of Materials Science: Materials in Electronics - The influence of microstructural changes from nano to micron grain size regime towards their structural, magnetic, electrical and microwave...     

Optical absorption of the Gd3Ga5O12 epitaxial films grown from Bi2O3-B2O3-CaO solution melt system

Single-crystalline garnet films with a stoichiometric composition of Gd₃Ga₅O₁₂ were grown for the first time by liquid phase epitaxy from a supercooled melt of the Bi₂O₃-B₂O₃-CaO solid solution system. The films exhibit no absorption bands in the visible spectral range.     

Mg共掺杂Gd3(Al,Ga)5O12:Ce晶体快发光的作用机理

Gd₃(Al,Ga)₅O₁₂:Ce (GAGG:Ce)闪烁体综合性能优异, 应用前景广阔。为加快GAGG的发光衰减速度, 本研究通过提拉法生长了Mg共掺的Gd₃(Al,Ga)₅O₁₂:Ce单晶。测试结果显示, 随着Mg²⁺掺杂浓度增加, 晶体的闪烁衰减速度加快, 光输出降低。传统解释认为, Mg²⁺通过电荷补偿作用将部分Ce³⁺转换成Ce⁴⁺, 后者的发光速度更快。本研究尝试从缺陷的形成与抑制的角度来讨论Mg改善GAGG:Ce晶体闪烁性能的作用机理。由于Ce的离子半径比Gd大, Ce离子掺入将导致发光中心Ce_Gd附近的晶格发生畸变。畸变结果为近邻的八面体格位空间变大, 反位缺陷将更容易在这些变大的八面体格位形成。最终每个发光中心Ce_Gd被四个反位缺陷Gd_Al包裹, 后者捕获载流子, 延缓从基体到发光中心的能量传递, 导致发光速度变慢。由于Mg的离子半径介于Gd和Al之间, Mg_Al将更容易在上述畸变的八面体格位形成, 这会抑制反位缺陷Gd_Al在发光中心Ce_Gd附近形成(或富集), 最终降低(甚至消除)反位缺陷对发光中心的不良影响。XEL测试结果显示, 随着Mg掺杂量增大, 与反位缺陷相关的发射峰强度变弱, 这可以证明Mg对反位缺陷有抑制作用。     

液相共沉淀法制备钆镓石榴石(Gd3Ga5O12,GGG)多晶原料

采用液相共沉淀的方法制备了钆镓石榴石(GGG)的多晶原料,给出了液相合成GGG多晶料的工艺步骤及其条件.就组分Gd2O3和Ga2O3的配比,滴定氧化物溶解的混合溶液使其沉淀的pH值,共沉淀后烧结温度以及烧结时间等影响GGG多晶料合成的重要因素进行了讨论.通过和固相合成GGG多晶料的条件比较可知,液相合成GGG多晶原料的工艺具有合成的GGG多晶料均匀充分、合成温度低与烧结时间短等优点.     

Influence of dislocations on the Kapitza conductance of copper at temperatures between 1 and 2 K

An experiment is described in which the dependence of the Kapitza conductance of copper to He II on the dislocation density near the interface is investigated in the temperature range of 1–2 K. A dependence is found, which has not been reported before, giving support to the dislocation -based theoretical models. The Kapitza conductance for a low-dislocation-density sample is shown to be an order of magnitude below the usual values, and within the range of validity of these theories.Supported by the Science Research Council.     

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.     

Temperature variation of the Lorenz ratio for high-purity gallium between 1.4 and 4.2 K

The electrical and thermal resistivity ( andW, respectively) of a large (1 cm diam) single crystal of high-purity gallium, oriented along theC axis, have been measured as a function of temperature over the range of 1.4–4.2K. Both andWT can be fitted to the same power law with an exponent close to three. As a consequence, the temperature-dependent part of the Lorenz ratio has a relatively constant value of about 0.4×10^–8V²·K^–2 for the entire range. Qualitative explanation for this is attempted in terms of the possible scattering mechanisms involved.