Colour centre in YAG: Cr3+ crystals

YAG crystals doped with 0.01 wt% Cr were grown by Czochralski method using 98% Ar + 2% H₂ protective atmosphere. Four colour varieties of the crystals were prepared in the dependence of the Al₂O₃ :Y₂O₃ ratio in the melt and the sign of electrical potential above the melt level. New absorption bands were attributed to the O⁻ centre (400 to 480 nm), F centre (357, 500, and 833 nm) or Cr³⁺ at another site than octahedral (455, and 612 nm).     

Nonstoichiometric defects in YAG and YAP

Spectroscopic properties of undoped YAG and YAP grown under reducing protective atmosphere were compared. Blue (with F centre) or brown (with 0⁻ centre) varieties both of the materials were prepared in the dependence of the Al₂O₃:Y₂O₃ ratio in the melt. Colour intensity depends on the H₂O:H₂ ratio in the protective atmosphere and on the sign of electrical potential above the melt level. The coloration may be easily removed by an annealing at > 1200–1400 °C. All the irregularities of the colourations or their remowing by annealing may be attributed to the presence of impurities.     

Colour Centre in Nd3+-doped Yttrium Aluminates

Spectroscopic and laser properties of YAG:Nd and YAP:Nd grown from the nonstoichiometric melts or those containing Mo and Fe impurity were compared. YAG:Nd grown from the melt with surplus of Y₂O₃ and annealed in H₂ at ∼ 1400 °C have an increased laser performance. Mo admixture in hydrogen annealed YAP:Nd laser crystals suppresses effectively the detrimental action of iron impurity.     

Temperature distribution in growing semi-transparent crystals (III). Conical liquid/solid interface

Temperature distribution in Al₂O₃ and YAG crystals grown by Czochralski method and in the melt layer adjacent to the liquid/solid interface of the conical shape was computed. The results were compared with the quality of the crystals. Using a highly absorbing material, temperature difference near the deeply submerged sharp interface decreases from the edge to the centre of a crystal and it is relatively great and unalterable in the case of a shallowly submerged blunt interface. Sharp interface show the crystals grown from the melt of the same composition (Al₂O₃ or accurately “stoichiometric” YAG), whereas the blunt one is typical of YAG grown from the melt of the “non-stoichiometric” composition, because its solidifying point is below the melting point of YAG phase.     

Spectral Properties of Oxide Crystals Free of Iron Ions

Al₂O₃ and YAG crystals were purified from traces of iron by the growth in reducing atmosphere. Luminescence output of such materials was substantially increased, but some undesirable properties as transient colour centre formation in YAG: Nd and low damage threshol of ruby laser rods were observed. Minimum concentration of iron ions which drastically change spectral properties of oxide crystals seems to be ⪅ 10⁻⁴ wt%.     

Some Aspects of Interactions in the Mo–W–Al2O3–H2 System: Formation of Polyoxides

The possibility of formation of molybdenum and tungsten polyoxides in the Mo–W–Al₂O₃–H₂ system at T = 2400 K and P = 1 bar in a controlled Ar + H₂ atmosphere has been investigated by the method of thermodynamic analysis. The formation of polyoxides is found to occur both due to the processes involving Al₂O₃ melt and in the absence of the latter. It is established that metals (Mo and W) and their mono-, di-, and even trioxides (in the latter case, mediated polymerization occurs) can be used as initial components to form polyoxides. It is shown that polyoxides themselves may interact with one of their main sources: Al₂O₃ melt.

     

Mechanisms of Interaction of Molybdenum and Tungsten Polyoxide with Aluminum Oxide Melt under Reducing Conditions

The main chemical reactions between Mo and W polyoxides and Al₂O₃ melt in a controlled Ar + H₂ atmosphere (T = 2400 K, P = 1 bar) during sapphire growth by horizontal directional solidification have been investigated. Under these thermodynamic conditions, the melt and products of its dissociative evaporation may actively react with the tungsten heater and molybdenum thermal screens of the crystallization system. It is shown that the polyoxides formed during evaporation do not directly interact with the melt; this interaction occurs only with participation of reagents exhibiting pronounced reducing properties (Al, H₂, H, WO, Al₂O, AlH, AlH₂, AlH₃). It is established that most of processes occur with participation of aluminum hydrides. A particular role of Mo(W) dioxide–W(Mo) polyoxide functional pairs in the interaction with the melt is determined.

     

The role of argon in yttrium aluminates

Spectral properties of YAG and YAP grown in vacuum or under Ar H₂ protective atmosphere were compared. The latter contained small amounts of argon which was detected by a laser mass spectral analysis. Ar impurity evokes brown colour of unannealed crystals and strenghtens anomalous red colour of annealed (Particularly in O₂) YAP crystals doped with Nd + Cr or Nd + Mo. The same red coloration of the doped YAGs is less stable.     

Chemical Processes in Al2O3-Mo and Al2O3-W systems in a weakly reducing atmosphere

Al₂O₃-Mo and Al₂O₃-W systems in a controlled Ar(95%) + H₂(5%) atmosphere at T = 2400 K and P = 1 bar have been calculated by the Monte Carlo method. It is established that the presence of hydrogen in these systems leads to the occurrence of OH, H₂O₂, HO₂, H₂O, AlOH, AlOOH, AlH, AlH₂, and AlH₃ components in the gas phase; aluminum hydrides are formed only through the interaction of hydrogen with melt evaporation products. The presence of reducing medium leads to a decrease in the free oxygen concentration by one to two orders of magnitude, which is expected to improve the quality of sapphire crystals.     

Cover Picture: Cryst. Res. Technol. 8/2009

The cover picture shows as grown garnet Y₃Al₅O₁₂ (YAG): Co, Si single crystals obtained by then Czochralski method with a diameter of 2 cm. From left till right: – YAG: 0.4 at.%Co, – YAG: 0.4 at.%Co, 0.2 at.%Si, – YAG: 0.8 at.%Co, 0.4 at.%Si. (see pages 823–833) (© 2009 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     

Temperature distribution in growing semi-transparent crystals (II). Comparison of theory with experiments

Calculated temperature difference between the liquid/solid interface and bulk of the melt was compared with the wide-angle scattering (celluae density) of Al₂O₃ and YAG crystals. Celluae free coloured crystals were grown using higher energy losses from the upper part of the crystal and the melt. Colourless crystals were successfully grown under opposite conditions. The results are in accordance with the calculated temperature differences which are proportional to the actual temperature gradient at the interface.     

Chemical processes in the Mo-W-Al2O3 system during sapphire crystal growth from melt

The main chemical reactions occurring in the Mo-W-Al₂O₃ system at T = 2400 K and P = 1 bar in an inert controlled atmosphere are considered. The mechanisms of Mo and W oxidation by the dissociative evaporation of melt products are determined. It is established that, at a formally low fraction of evaporating Al₂O₃, multiply repeated evaporation-condensation and oxidation-reduction cycles are unfavorable for the operation of W heater and Mo-containing elements of heating-unit elements. It is found that gaseous Mo oxides (MoO and MoO₃) can also affect the degradation of the W heater. A detailed study of the chemical processes should facilitate the optimization of the conditions of sapphire crystal growth from melt.     

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)     

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.     

Growth and properties of Lu3Al5O12-RE3+ single crystals

Optically transparent garnet single crystals were grown from Lu₃Al₅O₁₂ melts containing different RE³⁺ ions. The distribution coefficient of Nd³⁺ ions is found to be a function of the growth rate. Lu₂O₃–Al₂O₃ system studies have been partially carried out. Measurements of the lattice data, absorption spectra and comparison with some properties of Y₃Al₅O₁₂ single crystals have also been made.     

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.     

Growth and characterization of nanostructured Al2O3/YAG/ZrO2 hypereutectics with large surfaces under laser rapid solidification

The preparation of large bulk oxide eutectics with homogeneous and dense structure in nano-scale by melt growth method is a difficult challenge. Fully dense, homogeneous and crack-free ternary nanostructured Al₂O₃/YAG/ZrO₂ hypereutectic plate with large surface is successfully obtained by laser remelting. The hypereutectic in selected composition presents an ultra-fine eutectic-like microstructure consisting of alternating interpenetrating Al₂O₃, YAG and ZrO₂ lamellae with mean interphase spacing of about 150 nm, which is much smaller than the ternary eutectic composition grown at the same growth conditions. With the increase of laser scanning rate, the lamellar spacing is rapidly decreased. The minimum value obtained is 50 nm. The analysis indicates that the strong faceted growth behavior and cooperative branching of the component phases related with high entropies of fusion and large kinetic undercooling during laser rapid solidification are the primary formation reasons for the irregular eutectic growth morphology. Furthermore, the unique cellular microstructure with complex structure is also observed at high growth rate, and their formation mechanism and effect of the composition on the microstructure are discussed.     

Production of sapphire tubes for high-pressure sodium lamps using the stepanov method at high rates of growth

The process of the Stepanov growth of sapphire tubes is studied at the growth rates up to 20 mm · min⁻¹ in a Mo crucible with graphite succeptor. It is established that the main factor preventing the production of high-quality samples is contamination of the melt due to the gas transport reactions the growth chamber atmosphere and formation of oversaturated carbon solid solution in the Al₂O₃ matrix. The decomposition of this solid solution gives rise to the second-phase-particle precipitations. The production of highquality sapphire tubes at the growth rates up to 20 mm · min⁻¹ is possible with a careful elimination of water vapours and oxygen from the growth chamber atmosphere.     

Growth of sapphire by the kyropoulus method

Single crystals of leuko-sapphire (α-Al₂O₃) are grown from the melt by an improved Kyropoulos technique using Mo or W crucibles. The influence of the temperature field on the structural and optical quality has been studied. Best results were obtained when heat losses from crystal and melt are minimized.