Thermodynamic approach to the problem of A3B5 crystals with an optimum structural perfection

Thermodynamics of A³B⁵ semiconductor crystal, containing impurity and native point defects is considered. The main limit of the present analysis is the absence of a significant interaction between point defects at thermal equilibrium. It is shown that the difference of the chemical potentials of A³ and B⁵ components may serve as a structural sensitive thermodynamic function of a crystal. The optimization of the crystal perfection may be obtained when this function becomes zero. In the case of undoped A³B⁵ crystal such a situation corresponds to the ideal lattice perfection. In the case of doped crystal the effects of native point defects and their interaction with the impurity on the electronic properties of the crystal have to be reduced. The developed thermodynamic approach has been applied to the quantitative analysis of the available experimental data (GaAs, GaP, GaAs:Si and GaP:In). The results of the calculation are in reasonable agreement with the experiment.     

Thermal and optical properties of CuO–BaO–B2O3–P2O5 glasses

CuO-doped barium borophosphate glasses in a series of xCuO–(45 − x)BaO–10B₂O₃–45P₂O₅ in molar ratio with x = 0–15 mol% were prepared by a melt-quenching technique. All the glasses had excellent thermal stability against crystallization. Glass transition temperature, thermal expansion coefficient and molar volume decrease with increasing CuO concentration. The linear relationship between the absorption coefficient and CuO concentration exists for a peak wavelength in the transitions of ²A_1g → ²B_1g, ²B_2g → ²B_1g, ²E_g → ²B_1g. The relationship between the properties and glass structure evaluated by Raman spectroscopy is discussed.     

Phase equilibria of <Emphasis Type="Italic">A</Emphasis><Superscript>3</Superscript><Emphasis Type="Italic">B</Emphasis><Superscript>5</Superscript> quinary systems with an elastically strained solid phase

The contribution of the elastic component of the excess energy of mixing is determined for A³B⁵ quinary solid solutions. A relationship is derived for the activities of the components in an elastically strained solid phase of the quinary solid solutions. The correctness of the possible assumptions used in the calculation of the activity coefficients of the solid phase of a quinary solution is analyzed. The contact supercooling in the vicinity of the binary substrate isoperiods is calculated for a number of A³B⁵ quinary systems. It is shown that the negative contact supercoolings correspond to the thermodynamic instability boundaries of the quinary solid solutions.     

Solid-phase epitaxy for the formation of superlattices based on A3B5 compounds

To form superlattices based on A³B⁵ compounds, it is suggested to use solid-phase epitaxy (SPE) which includes recystallization of super-thin films of stoichiometric composition deposited by laser evaporation on single-crystal plates or films with the same or close composition. The recrystallization temperature is shown to be reduced considerably for the film grains with its curvature differing significantly from zero and which are in contact with their own single-crystal seed having practically zero curvature. The single-crystal multilayer films GaAs-(Al, Ga)As … GaAs were formed at 300°C. The recrystallization rate at this temperature was 40 Å/min. The autocatalytic action of the single-crystal seed strongly reduced the activation energy of the SPE process, which may be due to diffusion in solid phase.     

Intra- vs intermolecular hydrogen bonding. The crystal structure of 5-methyl-2-hydroxyacetophenone thiosemicarbazone monohydrate and salicylaldehyde-2-methylthiosemicarbazone monohydrate

The crystal structure of 5-methyl-acetophenonethiosemicarbazone monohydrate,A, and salicylaldehyde-2-methylthiosemicarbazone monohydrate,B, were determined using single crystal X-ray diffraction.A crystallizes in the monoclinic space groupC2/c, with lattice parametersa=14.161(2),b=15.753(1) Å,c=11.084(1) Å, β=112.59(1)° andZ=4, yielding a calculated density ofD _calc=1.352 mg/m³.B crystallizes in the triclinic space groupP1, witha=7.233(2) Å,b=7.371(2) Å,c=11.841(2) Å, α=82.77(2)°, β=78.33(2)°, γ=63.06(2)° andD _calc=1.371 mg/m³ forZ=2,. In bothA andB the immine nitrogen and the sulfur atom areanti with respect to N2-C8. WhileA presents the usual intramolecular six membered hydrogen bond ring,B has instead an intermolecular hydrogen bond between the hydroxy moiety of the salicyladehyde and a water molecule. AM1 calculations agree with the experimental conformations observed in both compounds.     

Electrical conductivity of GeTe glasses under hydrostatic pressure

The electrical conductivity of some GeTe bulk glasses has been measured between 10 and 80°C under hydrostatic pressure up to 3000 bar. The electrical conductivity (σ) of as-prepared, amorphous samples can be expressed by an equation: σ = A exp (?B/kT). For Ge₁₇Te₈₃ glass, the pressure dependences of the constants, A and B, are: (d ln A/dp) = ?3.2 × 10^?4 bar^?1 and (dB/dp) = ? 2.1 × 10^?5 eV · bar^?1. The results are analysed in terms of the low-mobility band model of Mott-CFO for amorphous semiconductors.     

Internal deformations in substitutional quaternary solid solution <Emphasis Type="Italic">A</Emphasis><Superscript>3</Superscript><Emphasis Type="Italic">B</Emphasis><Superscript>5</Superscript>

A model that describes the bond-length distributions in quaternary A³B⁵ solid solutions and allows one to consider large clusters using a minimum of computational resources is suggested. The analytical expression for the radial-distribution function is obtained. The results of the modeling describe the known experimental data well. For a number of solid solutions, the deformation energy is evaluated in the approximation of the valent force field. It is shown that the main contribution to the energy comes from the bond-length dispersion.     

On the growth mechanism of an A-B crystal: II. effect of dissociative adsorption of B2 molecules

In a previous treatment, the BCF theory which applies to a monatomic crystal has been extended to the case of an A-B crystal growing from the vapour according to a scheme A(g) + B(g) ? AB(cr).The effect of dissociative adsorption of diatomic gas molecules according to a scheme A(g) + 1/2 B₂(g)?AB(cr) is incorporated in the present work. The results obtained are as follows. (i) The growth rate for a constant driving force Δg = kT ln[P_A(P_B2)^1/2/P_A0(P_B201/2] varies with the partial pressures P_A and P_B2. (ii) In the limiting cases, P_A å P_B2 or P_A å P_B2, the major component is nearly under saturation and the growth rate is me$\text{r}$ely controlled by the minor component, which works as the so-called “growth unit”. (iii) Because of the associative desorption of B adatoms, the mean migration distance $\text{x}$_sB depends on the partial pressure P_B2. (iv) The theory is in good agreement with an experiment on the vapour growth of CdTe under controlled partial pressure of the constituent element.     

New hilgardite-group polyborate Pb2[B5O9]Br with high optical nonlinearity

New anhydrous lead borate Pb₂[B₅O₉]Br (sp. gr. Pnn2) was synthesized by the hydrothermal method. The second harmonic generation from polycrystalline samples of Pb₂[B₅O₉]Br is characterized by a higher signal than that observed in powdered LiB₃O₅. The crystal structure of the new hilgardite-group compound was refined by the Rietveld method. Analysis of the known orthorhombic polar varieties of hilgardites, including the new compound, showed that their boron-oxygen frameworks are occupied by the Pb²⁺, Ca²⁺, Eu²⁺, and Ba²⁺ cations; the Cl^?, Br^?, and OH^? anions; and water molecules in combinations determined by their sizes.     

On the structure of luminol sodium salts

The structures of Tamerit® (A) and Galavit® (B) pharmaceutical preparations have been solved by X-Ray single crystal and powder diffraction. These are luminol sodium salts possessing immunomodulatory and anti-inflammatory properties. It is shown that Tamerit® (A) is a hydrated salt, while Galavit® (B) is a mixture of two polymorphic modifications (B1 and B2) of anhydrous salt. Compound A is crystallized in a monoclinic system: a = 8.3429(4) Å, b = 22.0562(11) Å, c = 5.2825(2) Å, β = 99.893(3)°, V = 957.59(8) ų, and Z = 4; sp. gr. P2₁/c. Compound B1 is crystallized in a monoclinic system: a = 14.7157(18), b = 3.7029(19), c = 16.0233(15) Å, β = 116.682(13)°, V = 780.1(4) ų, and Z = 4; sp. gr. P2₁/c. Compound B2 is crystallized in an orthorhombic system: a = 27.7765(15) Å, b = 3.3980(19) Å, c = 8.1692(19) Å, V = 771.0(5) ų, and Z = 4; sp. gr. Pna2₁. The absence of phase transitions between the B1 and B2 polymorphs has been established by differential scanning calorimetry.     

X-ray diffraction investigation of 1-phenyl-3-isopropyl-5-(benzothiazol-2-yl)formazan

The crystal structure of 1-phenyl-3-isopropyl-5-(benzothiazol-2-yl)formazan is investigated using X-ray diffraction. The compound crystallizes in the form of two crystallographically independent molecules (A and B) in identical conformations that are stabilized by intermolecular hydrogen bonds. The intermolecular hydrogen bonds N-H…N (N…N, 2.892 and 2.939 Å) link molecules into AB dimers. Both molecules have a flattened structure, except for the isopropyl fragment. The bonds in the formazan chains are delocalized. Molecules A and B have close geometric characteristics.     

Crystal structure of Pr2B5 boride

The crystal structure of Pr₂B₅ boride of a new type was established by single-crystal X-ray diffraction analysis; sp. gr. C2/c, a = 15.1603(4) Å, b = 7.2771(2) Å, c = 7.3137(2) Å, β = 109.607(2)°, 2068 reflections, R _F = 0.041, R _w = 0.055. The Pr₂B₅ structure is similar to the Gd₂B₅ structure.     

On the temperature dependence of the enthalpies of mixing in liquid binary (Ag, Cu, Ni)-Sn alloys

The partial and integral enthalpies of mixing of liquid binary alloys in the systems Ag-Sn, Cu-Sn and Ni-Sn were determined at different temperatures from 500 to 1500 °C using drop calorimetry techniques. Generally, our measured values fit well to most of the hitherto reported results. The data were fitted using a Redlich-Kister polynomial and the binary interaction parameters were evaluated. For all three systems the enthalpy curves at various temperatures are discussed focusing on the association theory. C^ass,C^A₁,B₁,C^A₁,A_iB_j,C^B₁,A_iB_j, the interaction parameters and K_AiBj were determined. The general shape and supposed temperature dependence of the enthalpy of mixing in Ag-Sn and Cu-Sn could be confirmed. No significant temperature dependence could be found experimentally for Ni-Sn at temperatures above 1100 °C. Extrapolations into the metastable liquid regions based on the association theory, however, indicate a small drop of Δ_MixH with temperature. A second series of measurements limited to low concentrations of Ag, Cu, or Ni, respectively in Sn has been performed in order to determine the partial enthalpies of mixing at infinite dilution at 500, 600, 700, and 800 °C. Temperature dependence could be observed for all three binary systems, most developed for Ni-Sn.     

Crystal structure of 3-amino-5-acetyl-4,7-dihydro-6-methyl-4-(2-nitrophenyl)-2-cyanothieno[2,3-b]-pyridine and specific features of its molecular packing in crystal

The molecular and crystal structures of 3-amino-5-acetyl-4,7-dihydro-6-methyl-4-(2-nitrophenyl)-2-cyanothieno[2,3-b]-pyridine is determined by X-ray diffraction. The unit-cell parameters are as follows: a = 14.68(1) Å, b = 8.704(5) Å, c = 25.36(1) Å, V = 3241(6) ų, d _calcd = 1.453 g/cm³, Z = 8(1,1), and space group Pna2₁. Theo-nitrophenyl substituent is synperiplanar relative to the hydrogen atom at the C(4) atom of the heterocycle. Two crystallographically independent reference molecules A and B in the structure can be considered a pair of the enantiomers related by an approximate noncrystallographic center of inversion. The degree of approximation of this pseudocenter is discussed. It is shown that the structure can be described as a combination of two systems of translationally equal layers. The pseudosymmetry of the crystal structure and the possibility of forming a similar molecular packing with a higher crystallographic symmetry are considered.     

X-ray crystal structure of the complex of enniatin B with KNCS

The crystal structure of the complex of enniatin B, cyclo[-(L-MeVal-D-Hyi)₃-](C₃₃H₅₇N₃O₉), with KNCS is determined by X-ray diffraction [CuKαradiation, R = 0.0594 for 7925 reflections with I > 2 σ(I)]. The crystals belong to the space group P3, a = 24.448(5) Å, c = 23.578(5) Å, V = 12277(9) ų, and Z = 6. The unit cell contains 12 symmetrically independent molecules of the antibiotic, which are located on crystallographic threefold axes. The K⁺ ions are located on the threefold axes and are coordinated by the carbonyl oxygen atoms of the hydroxy acid or amino acid residues of the enniatin molecules to form prisms, twisted prisms, and antiprisms. All the independent enniatin molecules retain the principal conformational features revealed earlier in the structures of enniatin B and its complexes.     

Crystal structure of the new barium borate Ba5(BO3)2(B2O5)

The crystal structure of the new barium borate Ba₅(BO₃)₂(B₂O₅) is established (R = 0.0436). Single crystals were grown by spontaneous crystallization in the BaO-B₂O₃-Na₂O system using the flux method. This compound crystallizes in the orthorhombic system, sp. gr. P2₁2₁2₁; the unit-cell parameters are a = 9.590(2) Å, b = 16.659(3) Å, c = 22.919(6) Å, and Z = 12. The structure consists of coordination polyhedra of barium cations and the anionic groups [BO₃] (planar triangles) and [B₂O₅] (vertex-sharing double [BO₃] triangles), which form a pseudohexagonal framework. Melting of barium borate occurs by a peritectic reaction at 1170 ± 10°C.     

Interatomic force constants in AIIBIVCV2 chalcopyrite compounds

Force constants of the A^II C^V and B^IV C^V bonds in the A^IIB^IVC^V₂ compounds with chalcopyrite structure are estimated from experimental lattice vibration data using a simplified version of the Keating model. It is shown that the force constants depend exponentially on the bond length. The parameters of this relation are practically the same as those found for the A^IV A^IV, A^III B^V and A^II B^VI bonds in the elemental semiconductors and binary compounds and for the B^III C^VI bond in the A^IB^IIIC^VI₂ semiconductors.     

Influence of intrinsic defects on the electrical properties of AIBIIIC compounds

The electrical parameters of undoped melt grown A^IB^IIIC^VI₂ crystals are compared with predictions which can made considering the deviations from stoichiometry due to the incongruent evaporation of these compounds. It is shown that the electrical properties of all A^IB^IIIC compounds can be interpreted in a consistent manner if the electrical activity (donor or acceptor) of the intrinsic defects is considered in the covalent bonding model.     

Refractive index behavior in phosphate glass from the view point of B3+ and Al3+ coordination

Higher refractive index and higher Abbe value were obtained simultaneously at B₂O₃/Al₂O₃ = 1 in the system of 65P₂O₅–xAl₂O₃–(20 ? x)B₂O₃–10CaO–5Li₂O, although the refractive index generally shows a positive correlation with the dispersion. We investigated the molar volume and the molar refraction by Lorentz–Lorenz’s formula and also studied the coordination number of ¹¹B and ²⁷Al by MAS NMR. By the replacement of Al₂O₃ with B₂O₃, the molar refraction constantly decreased, but the molar volume was minimized at B₂O₃/Al₂O₃ = 1. The refractive index behavior of the glasses was mainly determined by the molar volume in the system. The coordination number of B³⁺ was only IV and it did not change if the composition was changed. On the other hand the coordinations IV, V and VI were observed for Al³⁺. The ratio of Al³⁺(VI) was maximized at B₂O₃/Al₂O₃ = 1. It is considered that the higher coordination of Al³⁺ brings the improvement of the packing and it leads to high refractive index.     

Arrhenius model for high-temperature glass-viscosity with a constant pre-exponential factor

A simplified form of the Arrhenius equation, ln η = A + B(x)/T, where η is the viscosity, T the temperature, x the composition vector, and A and B the Arrhenius coefficients, was fitted to glass-viscosity data for the processing temperature range (the range at which the viscosity is within 1-10³ Pa s) while setting A = constant and treating B(x) as a linear function of mass fractions of major components. Fitting the Arrhenius equation to over 550 viscosity data of commercial silicate glasses and approximately 1000 viscosity data of glasses for nuclear-waste glasses resulted in the A values of −11.35 and −11.48, respectively. The fraction of the variability (R²) value ranged from 0.92 to 0.99 for commercial glasses and was 0.98 for waste glasses. The Arrhenius models estimate viscosities for melts of commercial glasses containing 42-84 mass% SiO₂ within the temperature range of 1100-1550 °C and viscosity range of 5-400 Pa s and for waste glasses containing 32-60 mass% SiO₂ within the temperature range of 850-1450 °C and viscosity range of 0.4-250 Pa s.