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.     

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+).     

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.     

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.     

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.     

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.     

On the experimental reproducibility in measurements of the Kapitza conductance of copper between 1 and 2 K

The reproducibility of measurements of the Kapitza conductance between electropolished copper samples and He⁴ has been investigated at temperatures between 1.2 and 2.0 K. It is shown that the results are sensitive to the vacuum treatment to which the sample is subjected. It appears that after leaving samples for a long time under a vacuum of 10⁻⁷ mm Hg, obtained with a getter-ion pump, fairly reproducible data may result.     

Growth,Optical Absorption,and Photoluminescence of Epitaxial (Pb,Gd)3(Al,Ga)5O12:Ce3+,Eu3+ Films

Inorganic Materials - (Pb,Gd)3(Al,Ga)5O12:Ce3+,Eu3+ films have been grown by liquid-phase epitaxy from a supercooled high-temperature solution based on the PbO–B2O3 system. We have measured...     

On the magnetic field acting upon moving domain walls in (Bi,Yb)3(Fe,Ga)5O12 garnet ferrite films

The dependence of the domain wall velocity V on the magnetic field strength H in (111)-oriented (Bi, Yb)₃(Fe,Ga)₅O₁₂ single crystal garnet ferrite films is studied by the method of pulsed remagnetization at the point of angular momentum compensation. It is established that the acting field strength H is decreased by an effective internal magnetic field existing in the films studied.