Physico-chemical processes in single crystal growing of calcium aluminates by zone melting

The growing of single crystals of calcium aluminates of compositions 12 CaO · 7 Al₂O₂, CaO · Al₂O₃, and CaO · 2 Al₂O₃ by zone melting under vacuum of 10⁻⁵ mm Hg permitted to establish that some of the Al³⁺ ions in octahedral coordination have been driven back by the moving crystallization front. Energetically, this process can be represented on the basis of the viscous flow model with the activation energy of −45 kcal/mole. The possible mathematical models have been considered for the processes of preparation of single phase crystals of the aluminates mentioned above which take account of incongruent vaporization of the component oxides and refining of the melt from structural impurities by the moving crystallization front.     

Development of LuAG-based scintillator crystals – A review

A review of research and development of Lu₃Al₅O₁₂ (LuAG)-based single crystal scintillators is presented. Crystals of this type have been prepared by the micro-pulling down method at the initial stage of material screening and by Czochralski or Bridgman methods to obtain higher quality and larger size single crystals afterward. Several different activators, namely Ce³⁺, Pr³⁺, Yb³⁺ and Sc³⁺ have been reported in the literature and such doped LuAG single crystals have been extensively studied to understand a number of issues, including: the scintillation mechanism, underlying energy transfer and trapping processes including the nature and role of material defects involved in the scintillation process and their relation to manufacturing technology. Significant improvements in the understanding of aluminum garnet scintillators lead to the discovery of multicomponent garnet single crystal scintillators in 2011, which are described. These materials gave rise to new class of ultraefficient complex oxide scintillators, the light yield of which considerably exceeds the values achieved for the best Ce-doped orthosilicate scintillators.     

Crystal growth and optical properties of LuYAG:Ce single crystal

Crystals of Ce³⁺-doped (Lu_xY_1?x)₃Al₅O₁₂ (LuYAG) have been grown and studied by X-ray powder diffraction, emission spectroscopy, excitation spectroscopy and X-ray excited fluorescence spectroscopy. The X-ray powder diffraction pattern revealed that the as-grown LuYAG:Ce crystal possessed the garnet structure. Compared with Ce³⁺-doped Y₃Al₅O₁₂ (YAG), the absorption bands associated with the 4f–5d transition shifted to shorter wavelengths, the emission band that originated from a transition from the lowest 5d level to the ²F ground state of the Ce³⁺ ions shifted to the blue, which was probably due to a larger Stokes shift of the emission, and the reduction of relative intensity of antisite defect emission in the X-ray excited fluorescence spectra revealed that introducing Lu ions into YAG could reduce the antisite defect.     

Refractive indices of laser nanocrystalline ceramics based on Y3Al5O12

Refractive indices of the nanocrystalline Y₃Al₅O₁₂ ceramic and a garnet single crystal of the same composition have been measured. In the visible and near IR range (0.4–1.064 μm), the prism method was used; in the medium IR range (2–6.2 μm), the interference method with the use of thin plates was applied. The refractive indices of these crystalline materials are practically the same over the entire spectral range studied and are described by the approximate formula proposed earlier for a single crystal. The parameters of the continuos-wave lasing in the nanocrystalline Y₃Al₅O₁₂ ceramic doped with Nd³⁺ and Yb³⁺ ions measured recently are presented.     

Determination of Concentration of Rare Earth Ions Yb3+, Er3+, Tb3+, Ho3+, Tm3+ in Monocrystals of Yttriumaluminum Garnets on Basis of Magnetic Susceptibility

Crystals of solid solutions (R_xY_1-x)₃Al₅O₁₂ (where R is rare earth ion Er³⁺, Yb³⁺, Tb³⁺, Ho³⁺, Tm³⁺) with garnet structure were grown. The temperature dependencies of magnetic susceptibility for these crystals were obtained. On the basis of measurement of magnetic susceptibility a non-destructive technique for determining the concentration of rare earth ions in yttrium-aluminum garnets was developed.     

Nd3+:Y3Al5O12 laser ceramics: Influence of the size of yttrium oxide particles on sintering

The influence of the size of Y₂O₃ powder particles on the structure formation and densification of Nd³⁺:Y₃Al₅O₁₂ laser ceramics has been studied. It is shown that the use of 50- and 100-nm yttrium oxide particles makes it possible to synthesize single-phase yttrium aluminum garnet at temperatures of 1200 and 1500°C, respectively, whereas in the case of 5000-nm yttrium oxide particles 2-h exposure at a temperature of 1500°C yields only 80 wt % of the Nd³⁺:Y₃Al₅O₁₂ phase. Bulk swelling of pressed samples during sintering of 2.94Y₂O₃-0.06Nd₂O₃-5Al₂O₃ powders with the size ratio of the initial particles R(Al₂O₃/Y₂O₃) ～ 5 is observed. The application of different-sized powders (R ～ 2.5) provides quantitative ratios between phases in the 3Y₂O₃-5Al₂O₃ system at which shrinkage in a temperature range of 20–1500°C is dominant. Laser ceramics 0–2 at % Nd³⁺:Y₃Al₅O₁₂ have been obtained by the solid-phase sintering of oxide powders (R ～ 2.5). The slope efficiency for 1 at % Nd³⁺:Y₃Al₅O₁₂ laser ceramics is found to be 33%.

     

Growth and scintillation properties of Pr doped Gd3(Ga,Al)5O12 single crystals

Pr:Gd₃(Ga,Al)₅O₁₂ single crystals were grown by the micro-pulling down (μ-PD) method. All grown crystals were greenish and transparent with 3.0 mm in diameter, 15–30 mm in length. Neither visible inclusions nor cracks were observed. Luminescence and scintillation properties were measured. The substitution at the Al³⁺ sites by Ga³⁺ in garnet structure has been studied. In these crystals, Pr³⁺ 5d–4f emission is observed with 340 nm wavelength. Pr1%:Gd₃Ga₃Al₂O₁₂ shows highest emission intensity. The light yield of Pr:Gd₃Ga₃Al₂O₁₂ sample with 3 mmφ×1 mm size was around 4500 photon/MeV. Scintillation decay time was 7.9 ns (0.5%), 46 ns (0.7%) and 214 ns (98.8%).     

SQUID magnetometry,EPR and XPS characterization of Y3Al5O12: Co,Si single crystals

Single crystals of garnet Y₃Al₅O₁₂ (YAG): Co and Co, Si were grown by Czochralski method as materials for optoelectronic applications. Silicon doping is responsible for the change of the Co³⁺ ions into Co²⁺ in order to keep charge neutrality of the material and for a coloration of materials. Magnetization and Electron Paramagnetic Resonance measurements show that the most cobalt ions in the silicon co‐doped crystals were Co²⁺ in octahedral positions. Estimated concentration of the Co²⁺ ions were close to Si⁴⁺ nominal concentration. A little excess of Co²⁺ may be ascribed to defects present in the crystals. For the crystal doped only with Co, the concentration of Co²⁺ was about 13% of the nominal Co amount and might be caused by crystal defects. X‐ray Photoelectron Spectroscopy measurements exhibit aluminium deficiency. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence of annealed LLGG:Cr3+ crystals under high pressure

High pressure photoluminescence has been used for analysis of lattice disorder in La_2.7Lu_2.29Cr_0.01Ga₃O₁₂ and La_2.32Lu_2.59Cr_0.02Ga_3.07O₁₂ crystals. Photoluminescence of samples before annealing and those annealed for 5 h in oxygen and hydrogen atmosphere at 923 K has been measured. The pressure dependence of Cr³⁺ luminescence has been used to obtain the crystal field distribution (lattice disorder) and its dependence on the thermal treatment. The distribution energy of the ⁴T₂ state has been considered as a measure of disorder. Annealing in oxygen has been found to reduce the energies of ⁴T₂ states for all sites, and annealing in hydrogen – to increase these energies.     

EPR spectra of Cr3+-doped LuNbO4 crystals

Ferroelastic LuNbO₄ single crystals containing 0.3 at. % Cr³⁺ ions have been grown by the floating zone technique, and their EPR spectra have been studied at a frequency of 9.8 GHz at room temperature. The lines on the spectra are due to the transitions caused by three paramagnetic centers formed as a result of the replacement of one isovalent position of a Lu₃₊ ion and two nonisovalent positions of Nb₅₊ions by Cr₃₊-ions. As a result of twinning, the line number is doubled and four principal directions arise along which the same spectra are obtained. The spectra of these centers were described by a spin Hamiltonian with S = 3/2, the D and E parameters ranging from 0.024 to 0.17 cm^?1, and the g-factors g ∥ = 1.75–2.20 and g ⊥ = 1.90–2.13.     

Characterization of large‐sized Nd:YAG single crystals grown by horizontal directional solidification

Nd³⁺‐doped Y₃Al₅O₁₂ single crystals have been grown by the horizontal directional solidification (HDS) method in different thermal zone. The Grashof (G_r), Prandtl (P_r), Marangoni (M_a) and Rayleigh (R_a) numbers of melt in HDS system have been discussed for our experimental system to understand the mechanism of melt flow patterns and concentration gradient of dopant. The concentration gradient of Nd³⁺ ions was explained with melt flow processes during crystal growth in different thermal zone, and results indicated that high growth temperature will be helpful for uniformity of dopant in HDS‐grown single crystal. The main microscopic growth defects such as bubbles and irregular inclusions in HDS‐grown Nd:YAG crystals were observed, and the causes were discussed as well. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Y2O3-Al2O3-Nd2O3 phase diagram and the growth of (Y,Nd)3 Al5O12 single crystals

The optimum compositions of the melts used for the growth of yttrium-aluminum garnet (YAG) single crystals with different neodymium contents are determined using the phase diagram of the ternary system Y₂O₃-Al₂O₃-Nd₂O₃ with the binary sections Y₃Al₅O₁₂-Nd₂O₃ and Y₃Al₅O₁₂-Nd₃Al₅O₁₂. A number of melt compositions characterized by one garnet phase, namely, (Y,Nd)₃Al₅O₁₂, are established. Single crystals of yttrium-aluminum garnets with a high content of the activator (up to 2.6 wt % Nd) are grown by the Czochralski method. __________ Translated from Kristallografiya, Vol. 48, No. 5, 2003, pp. 945–949. Original Russian Text Copyright ? 2003 by Soboleva, Chirkin. Dedicated to the 60th Anniversary of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences     

The role of Al3+ ions in alumino borate glasses as revealed by molar volume,refractive index and microhardness

An attempt has been made determine the types of Al³⁺ structural units in potassium alumino borate glass. Therefore a number of investigations have been carried out on the relation between the composition of the glasses and their properties. The density, refractive index, and hardness of a number of compositions in which Al₂O₃ replaces B₂O₃ at constant K₂O were measured. It appears that the replacement of B₂O₃is not entirely without some consequencies.The concentration of non-bridging oxygen ions increases with the introduction of Al₂O₃.Non bridging oxygen ions start aappearing at alkali concentration of about 0.2 mol K₂O.     

Gas bubble associated mass transport in Bridgman-Stockbarger melt growth system

Experimental evidence has been obtained showing that gas bubbles may be responsible for melt stirring in Bridgman-Stockbarger growth system which results in Pfann type impurity distribution profiles along the crystal length. The hypothesis of the gas bubble associated mass transfer mechanism is supported by the production of Y₃Al₅O₁₂–Nd³⁺ single crystals under conditions which eliminate or limit gas bubble nucleation in the melts with Nd³⁺ distribution profiles similar to those generally observed in melt growth systems where the mass transport in the melts is limited to diffusion.     

New data on the physical properties of Y3Al5O12-based nanocrystalline laser ceramics

Microhardness and fracture toughness of highly transparent Y₃Al₅O₁₂-and Y₃Al₅O₁₂: Nd³⁺-based nanocrystalline ceramics are measured for the first time. For the Y₃Al₅O₁₂: Nd³⁺ laser ceramics, the use of a longitudinal scheme with a diode-laser pumping at a wavelength of 1.3186 mm (⁴ F _3/2 → ⁴ I _13/2 channel) enabled one to attain an output power of continuous-wave lasing of ～3.7 W with 35% efficiency.     

Synthesis,crystal growth and second harmonic generation properties of trivalent rare-earth-doped non-linear tungsten–bronze-type structure Ba2Na1−3xRExNb5O15 (RE=Sc,Y, La,Gd, Yb and Lu)

A new compound filled tungsten–bronze-type structure Ba₂Na_1−3xRE_xNb₅O₁₅ (trivalent rare-earth ions: RE³⁺=Sc³⁺, Y³⁺, La³⁺, Gd³⁺, Yb³⁺ and Lu³⁺, with x=0.02) has been prepared by selecting RE³⁺ ions without any absorption in the visible range. The effect of rare-earth addition on micro-twin formation, that is to say, tetragonal–orthorhombic ferroelastic phase transition in barium sodium niobate was investigated and micro-twin formation could be suppressed by doping of smaller RE³⁺ ions such as Yb³⁺ and Lu³⁺. All the samples exhibit an intense second harmonic generation signal under tunable IR pumping laser source, due to both the high values of the non-linear optical coefficients and the absence of absorption of additional RE ions in the visible range.     

Luminescence properties of Y3Al5O12:Eu3+-coated submicron SiO2 particles

Silica submicron spherical particles coated with an yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) layer doped with Eu³⁺ were prepared by the sol–gel method. The structure and morphology of samples determined by the X-ray powder diffraction measurements and transmission electron microscope images, respectively, indicated that well-crystallized garnet nanocrystallites were formed with successive coating cycles. Similar trends were deduced from the evolution of the luminescence spectra. The ratio of integrated intensities of the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions was used to analyze the structural variations in the surroundings of the Eu³⁺ ion. The effect of coating was analyzed by comparing the luminescence properties of the Y₃Al₅O₁₂:Eu³⁺ nanocrystalline powders and composite Y₃Al₅O₁₂:Eu³⁺/SiO₂ materials.     

The effect of B2O3 on the luminescent properties of Eu ion-doped aluminoborosilicate glasses

A series of Eu ion-doped aluminoborosilicate glasses was prepared by melt-quenching method and characterized by spectroscopic techniques, including Fourier transform infrared (FTIR), absorption, photoluminescence (PL), and electron paramagnetic resonance (EPR). The FTIR spectra showed that various glass structures formed when Al₂O₃ was partially replaced by B₂O₃. The PL characterization revealed that the emission intensity of Eu²⁺ ions firstly increased and then decreased with an increasing amount of Al₂O₃ replaced by B₂O₃. Meanwhile, the emission intensity ratio of [Eu²⁺]/[Eu³⁺] also followed the same trend. The EPR spectra confirmed the concentration variation of Eu²⁺ ions in the glass samples, which agreed well with the PL results. The possible mechanism of the effect of the glass network structure on the reduction behavior of Eu ions is discussed.     

Optical properties and valence change of samarium ions in a sol–gel Al2O3–B2O3–SiO2 glass by femtosecond laser irradiation

10Al₂O₃–5B₂O₃–85SiO₂–xSm₂O₃ glasses were prepared by the sol–gel method. The emission spectra of the glasses indicate that the quench concentration of the Sm³⁺ ions is about 0.2 mol%. The emission spectra of the glasses after high-temperature treatment with H₂ gas exhibit the coexistence of the Sm³⁺ and Sm²⁺ ions. We observed the strong emission line of the Sm²⁺ ions and the emission band of the non-bridging oxygen hole center when the glasses were exposed to a femtosecond laser. It indicates that some Sm³⁺ ions were reduced to Sm²⁺ ions by femtosecond laser pulses and non-bridging oxygen hole centers were formed. The ⁵D₀–⁷F₀ emission line of the Sm²⁺ ions by femtosecond laser irradiation shows a red shift, compared with the emission of the Sm²⁺ ions by reduction with H₂ gas. The strong absorption band and weak, sharp absorption lines in the range from the UV to IR come from charge transfer and the transition from the ⁶H_5/2 state to the various excited states of the Sm³⁺ ions. The reduction mechanism of Sm³⁺ ions is discussed.     

Gd3Ga5O12:Nd3+ crystals for a continuous-wave diode-pumped laser operating in 4F3/2 → 4I11/2 and 4F3/2 → 4I13/2 channels

Garnet crystals of the composition Gd₃Ga₅O₁₂:Nd³⁺ (concentration series C_Nd = 1–10 at. %) were grown from flux. In terms of spectroscopy, these crystals, unlike those grown from melts, form medium with a single activator center. For the first time, continuous-wave lasing was excited by diode pumping with the use of Gd₃Ga₅O₁₂:Nd³⁺ crystals at the wavelengths λ₃ = 1.3315 and λ₄ = 1.3370 μm of the ⁴F_3/2 → ⁴I_13/2 channel and also the simultaneous generation at two wavelengths, λ₁ = 1.0621 and λ₂ = 1.0600 μm, of the ⁴F_3/2 → ⁴I_11/2 channel.