Phase Equilibrium of the Ga–Al–Sb system

Employing the method of liquid-phase epitaxy (LPE) solid-solutions of Ga_1–xAl_xAl_xSb (0 ≦ x ≦ 0.8) have been obtained. The dependence of Sb solubility on Al concentration in the liquid phase at 403°C, 452°C, 500°C has been established. The dependence of AlSb concentration in the solid phase on the composition of the liquid phase has been investigated at 452°C. Using the chemical constants equilibrium method, the phase equilibrium of the Ga–Al–Sb system in the region of liquid phase composition near the Ga-rich corner of the phase diagram has been calculated. The comparison of experimental and calculated data for the liquid and solid phases shows their agreement within the limits of experimental error.     

Epitaxial Deposition of GaAs in the Ga (CH3)3?AsH3?H2-System (IV) Thermodynamic and Kinetic Considerations

For reactions taking place between the species Ga, As₂, As₄, AsH₃, and H₂, the equilibrium constants were calculated. The discussion indicates that factors other than thermodynamic considerations significantly influence the deposition process. Kinetic investigations demonstrate the importance of the As:Ga proportion that has to apply for perfect layer growth. The mechanism of deposition is in correspondence with a Langmuir-Rideal Model. For high substrate temperatures a homogeneous reaction in the vapour phase can-not be excluded.     

Metallurgy of bronze-process A-15 superconducting composites

Several aspects of the metallurgy of multifilamentary composites of Nb₃Sn, V₃Ga, and other A-15 superconductors produced by the bronze process are reviewed. Among the factors discussed are phase diagrams, reaction kinetics, alloying effects, and microstructure-properties relations.     

Phase Diagrams of Semiconducting Compounds (III) Thermodynamic Calculation of the Ternary Melting Diagram of Ga?Al?As

The ternary phase diagram of Ga Al As has been calculated by VIELAND 'S method. The activity coefficients are determined using DARKEN 'S quadratic formalism for a ternary liquid in equilibrium with a regular solid solution. The agreement of calculation and experimental results is examinated using data from literature and solution measurements and liquid phase epitaxy experiments of this paper.     

Low-temperature Region of the In–Se System

A study of the low-temperature region of the In–Se system has been performed by means of isothermal annealing of diffusion couples as well as by X-ray, microscopic and microprobe analyses. Four binary compounds have been observed – In₄Se₃, InSe, In₆Se₇ and In₂Se₃. Original experimental results about the crystal structure and electrical conductivities of the binary compounds have been compared with available literature data or calculated values. A DSC study of the compound InSe has revealed that it melts peritecticly at about 618 °C. The diffusion controlled transition from one In-Se compound to another can be achieved without kinetic difficulties, following the phase arrangement from the equilibrium phase diagram. There are indications that small deviations from the exact stoichiometric ratios are possible.     

Crystal structure of La3Nb0.5Ga5.5O14 at 20 K

The atomic structure of La₃Nb_0.5Ga_5.5O₁₄ single crystals (space group P321, R = 2.21%, R _w = 2.31%) at 20 K was determined using X-ray diffraction. A comparative analysis of the structural characteristics determined at temperatures of 293 and 20 K did not reveal a noticeable redistribution of bond valences that could be considered precursors of a possible phase transition accompanied by the reduction of the symmetry of the crystal. The assumption was made that the anomalous behavior of the permittivity e ₃₃ of the La₃Nb_0.5Ga_5.5O₁₄ single crystals is associated with the decrease in the possible relative displacements of the Ga(1) and Nb atoms occupying the cation position with point symmetry 32 in a 1: 1 ratio.     

Study of phase transitions and the incommensurate phase in ACBX 4 crystals by the Monte Carlo method

The statistical model of successive phase transitions in the family of crystals described by the general formula ACBX ₄ has been studied. It is assumed that tetrahedral BX ₄ groups in the disordered tetragonal phase can be located in four equally probable equilibrium positions. Five representatives with different sequences of phase transitions in this family of crystals are considered. The effective equilibrium constants of interactions between tetrahedral groups are calculated within the framework of the electrostatic model. The corresponding phase diagrams and the thermodynamic characteristics of the phase transitions are studied by the Monte Carlo method. The experimental data provided the establishment of the sequence of phase transitions in Rb₂ZnCl₄ and K₂SeO₄ crystals with the formation of an intermediate incommensurate phase. The character of the structure modulation in this phase is established. The calculated thermodynamic characteristics of the phase transitions agree quite well with the experimental data for all the crystals under consideration.     

A thermodynamic factor influencing the growth rate and purity of epitaxial layers in the Ga-AsCl3-H2 system

Thermodynamic calculations of the Ga-AsCl₃-H₂ VPE system were expanded to incorporate the incompleteness of the reaction between the source Ga and gas phase. As a result, a saturation temperature was obtained, at which the gas phase reaches equilibrium with the substrate GaAs. The effect of incomplete contact between the gas and the source Ga was taken into account in the calculation by a flow efficiency which defined the hypothetical fraction of the gas which made complete contact with the source Ga. The limitation of the kinetics was described by a reaction efficiency which was incorporated into the reaction constant. The saturation temperature was measured experimentally for two reactors with different geometry. Values of the two efficiencies were derived and their dependence on growth conditions were also examined. The growth rate and undoped impurity level were found to be related in a simpler and perhaps more fundamental way to the saturation temperature than to other growth parameters.     

On the chemical transport of some niobium oxides with TeCl4 (II). The transport behaviour of the equilibrium NbO2.417 to NbO2.5

A report is given on the transport behaviour of the equilibrium phases NbO_2.417 to NbO_2.483 and of niobium pentoxide during chemical transport with TeCl₄. – It became apparent that phase-pure NbO_2.417 and Nb₂O₅ are deposited from the investigated equilibrium phases. Chemical transport of the phases NbO_2.453 to NbO_2.483 produces a mixture of several phases. This indicates a strong inhibition of the equilibration in the starting solid. Chemical transport of the niobium pentoxide caused the deposition of monocrystals of the upper and lower phase boundary. The experimental behaviour agrees very well with the calculated results. Monocrystals of the niobium pentoxide forms H, M, N, B and P are deposited in a well identifiable manner by variation of the temperatures (T₂, T₁).     

Phase equilibria in the Ga–P system. Calculation of activities and vapour pressures using a simple asymmetrical solution model

KRUPKOWSKI'S expression of activity coefficients has been applied to the liquid binary mixed phase of Ga–P. An interaction parameter of α = 1909 – 4.107 T and an asymmetry coefficient of m = 1.448 were determined from liquidus data and an entropy of fusion of GaP of 16.8 e.u. per mole at 1743 K. The activity coefficient of phosphorus along the liquidus curve shows a minimum at x_P = 0.032. There is an excellent agreement between calculated and experimental activities and vapour pressures over the whole controllable range of concentrations of the liquid phase. A total phosphorus pressure of 30.8 atms was calculated for the melting point of GaP.     

Diffusion Paths for Interstitial Impurities in Different Polymorphic Modifications of Niobium Silicide Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>

Structural models of the α, β, and γ modifications of Nb₅Si₃ silicides, which are used as a reinforcing phase in composites obtained in situ based on the Nb?Si system, have been constructed by computer simulation methods. A geometric analysis of unit cells is performed using the H-poisk program to estimate the voids existing in the structures. The results of measuring the number of voids and their sizes are reported. A conclusion about possible diffusion paths of interstitial boron, carbon, nitrogen and oxygen atoms is drawn based on the calculation results, and the solubility of these impurities in the structure of each Nb₅Si₃ modification is estimated.     

Crystallization behaviours ofCo100−1(x+y)NbxBy amorphous alloys

The crystallization behaviours of Co_100?1(x+y)Nb_xB_y amorphous alloys were investigated by means of differential thermal analysis and a conventional X-ray diffractometer. The crystallization sequences are discussed in terms of the equilibrium phase diagram of the ternary alloy system.Assuming crystallization occurs as a result of nucleation and growth, the stabilizing effect of eutectic phase separation on the crystallization is shown by introducing the effective free energy of the critical nucleus.     

Effective kinetic coefficients for some binary metallic alloys within the small and finite supercooling region

The normal crystallization kineties concerning the base of In, Rb, Ga some binary metallic melts either within the small supercoolings (Δ T ≦ 1 K) or finite ones (ΔT ≦ 20 ÷ 60 K) has been considered. When crystallizing the binary alloys: In + 2.3 at.% Pb; Rb + 10 at.% K; Ga + 2 at.% Sn; Ga + (2; 4; 8) at.% In; Ga + (0.5; 3.5) at.% Zn; Ga + (5; 8; 10) at.% Zn very dilute binary solid solutions are supposed to be formed. The solid solutions have the B-component distribution coefficients which are either much less or less compared with unit, i.e. k_B ≦ 1, k_B < 1 in all the layers of diphase transition region separating the melt from the crystalline phase. By virtue of fluctuation theory of normal crystal growth within the small and finite supercoolings' region for the above-mentioned binary metallic melt some effective kinetic coefficients have been calculated. A good agreement between theoretical effective kinetic coefficients and available experimental ones in the case of given binary systems has been established.     

Experimental Determination and Thermodynamic Calcultions of the Relationship between Vapour Phase Composition and Mixed Crystal Composition in the Ga-HCI-PH3-AsH3-(NH3)-H2 System

The relationship between the composition of GaAs_1–xP_x layers and the composition of the vapour phase has been investigated experimentally and theoretically. Using KIRWAN'S equilibrium constants excellent agreement between calculated and experimental data is obtained over the whole composition range. Different models and parameters for the calculation of the soild solution activity coefficients result in only small differences for the calculated alloy composition so that no decision can be made from the experiments for one model to be preferred. Increasing addition of NH₃ to the vapour phase up to concentrations exceeding those of the crystal forming gases by an order of magnitude has no influence on the alloy composition, although the nitrogen concentration in the layers increases as deduced from the shift of the wavelength of the maximum photoluminescence intensity.     

On the kinetics of nitrogen incorporation in GaP LPE layers using NH3 vapour doping

The partial substitution of N for P in GaP has a considerable effect on the luminescence quantum efficiency of GaP epitactic layers. The incorporation of N in GaP liquid phase epitactic layers using NH₃ vapour doping has been studied at different H₂ partial pressures in argon gas. It is shown that the incorporation of N in different ambients is conveniently described by just one parameter. The reaction of NH₃ with Ga is found to be an equilibrium process. The N concentration in the layer is directly proportional to the GaN concentration in the melt. From the correlation of the dissociation of NH₃ with the formation of GaN it is concluded that the NH₃ pressure, as measured in the exit gas, is not representative of the NH₃ pressure at the melt. The dissociation of NH₃ is found to be strongly retarded by H₂ owing to adsorption of H₂ on the surface of reactor materials which catalyse the dissociation.     

Ordering in BaMg1/3Ta1/3Nb1/3O3 ceramics: An X-ray Rietveld analysis

Structure analysis of polycrystalline BaMg_1/3Ta_1/3Nb_1/3O₃ ceramics has been carried out using the Rietveld technique, wherein the B cations (Mg, Ta, and Nb) have the same occupancy with a size difference and charge imbalance. The compound has an ordered trigonal structure as well as a disordered cubic phase depending on the preparation temperature. The weight percentage of the ordered phase is found to increase with a rise in the calcination temperature. Published in Russian in Kristallografiya, 2006, Vol. 51, No. 2, pp. 259–263. The text was submitted by the authors in English.     

Czochralski growth and characterization of Dy: La3Ga5.5Nb0.5O14 single crystals

Dysprosium (Dy) doped La₃Ga_5.5Nb_0.5O₁₄ single crystals were grown by the traditional Czochralski method along z‐axis. The structure of the crystal has been studied by X‐ray powder diffraction method, and the unit‐cell parameters are calculated to be a=8.22070 Å, c=5.12533 Å and V=299.965 ų. The segregation coefficient of Dy³⁺ in La₃Ga_5.5Nb_0.5O₁₄ crystal was measured by X‐ray fluorescence analysis. For 1 mol% doping level in the melt, the distribution coefficient of Dy³⁺ was determined to be 0.341 wt%. Specific heat, thermal expansion and transmission spectrum of Dy: La₃Ga_5.5Nb_0.5O₁₄ single crystals have been measured. The fluorescence spectra of Dy³⁺: La₃Ga_5.5Nb_0.5O₁₄ crystals were measured at room temperature, and there were four emission transitions occurring at 479, 576, 662 and 754 nm, respectively. The fluorescent lifetimes measurement results show 1.0% Dy: La₃Ga_5.5Nb_0.5O₁₄ possesses shorter fluorescence decay time (303.4 μs) than does 1.0%Dy:LGS (436.12 μs). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of silicon on the crystallographic phases of the niobium-gallium system studied by x-ray diffraction methods

The influence of silicon on the crystallographic phases of the binary niobium-gallium system is studied by X-ray diffraction methods. The samples were prepared by rapid quenching. Silicon substituted gallium in all phases of the binary system. It exists a complete pseudobinary solid solution Nb₅Ga₃-Nb₅Si₃. The maximum degree of substitution in the A 15 phase Nb₃Ga_1–xSi_x is x = 0.5. The presence of two A 15 phases with different cell parameters indicates the existence of a miscibility gap. Contrary to the expections the supercondusting transition temperatures decreases with an increasing degree of substitution.     

Conversion of non-stoichiometry of LiNbO3 to constitutional stoichiometry by impurity doping

Several chemical components exist in the oxide melt, including ionic species. During crystal growth these are partitioned into the solid phase with their own equilibrium partitioning coefficients. Therefore, they are usually segregated, even at the congruent composition. However, when a compound has a stoichiometric structure at the congruent composition, the equilibrium partitioning coefficients of all chemical components become unity and segregation does not occur. Segregation of ionic species was observed at the binary congruent composition of LiNbO₃ because of its non-stoichiometric structure. On the other hand, the segregation of ionic species did not occur at the ternary congruent composition of MgO-doped LiNbO₃ (Li₂O:Nb₂O₅:MgO=45.5:50:4.5). This demonstrates that non-stoichiometry of congruent LiNbO₃ was converted to the constitutional stoichiometry at the ternary congruent composition by MgO doping.     

Thermodynamic Calculation of the Alloy Composition of GaxIn1−xAs Grown in the Trichloride System Using Solid Sources

The dependence of the Ga_xIn_1−xAs alloy composition as a function of the experimental input variables is calculated for a growth system using AsCl₃ and solid GaAs and InAs sources. The results are similar to those earlier obtained for growth in the hydride system. Additional AsCl₃ introduced downstream the source regions shifts the alloy composition towards higher GaAs content.