Morphology of polycrystalline cassiterite films

Polycrystalline cassiterite films have been grown by the hydropyrolytic method from a 10(H₂O) + 5(SnCl₂ · 2H₂O) solution (in weight fractions) on corundum substrates. The crystallization regularities are considered and a comparative analysis of the properties of natural and artificial cassiterite crystals is performed. The surface morphology is investigated and the size of crystalline grains is determined by scanning electron microscopy. X-ray microprobe analysis showed that all films contain tin and oxygen atoms in a ratio corresponding (within the experimental error) to the chemical formula of tin dioxide. It is established that the surface morphology of cassiterite films is characterized by both single crystallites and aggregates of two or more crystals typical of twins. It is suggest that doping can efficiently be used to control the concentration of twins and the stability of their formation.     

Surface and Magnetic Orientation in a Type II Nematic Lyomesophase

Abstract

A type II nematic lyomesophase formed by a quaternary system (K decanoate/water/decanol/KCI) has been studied by X-ray diffraction (XD) and optical microscopy (OM) at room temperature. Samples in capillaries of various thicknesses, materials and geometries have been studied under the influence of wall effects, residual magnetic orientation and orientation in presence of magnetic fields. Results evidence that surface orientation extend typically up to 0.5 mm. The planar diffracting units have a characteristic distance in the director direction of 36 Å. The analysis of the compromise between surface and magnetic orientation by XD allowed to determine a critical field of 200 G for a 0.5 mm thickness; the elastic constant is estimated as 10^?-⁷ dynes. Analised by OM, the sample is axially positive, but shows also a weak biaxiality.     

Surface crystallization of β-BaB2O4 phase using a CO2 laser source

This letter describes decrystallization process of the β-BaB₂O₄ phase on the surface of 63BaO-26B₂O₃-11TiO₂ glassy sample when irradiated by a CO₂ laser beam for 30-40 s, using a laser power of ≈1.7 W. An analysis by optical microscopy of the irradiated region showed that the glass surface contained crystallites with the same morphology as those observed after traditional crystallization in an electric furnace. However, the crystal growth rate and the crystallite sizes appeared to be higher than those obtained by heating in an electric furnace. X-ray diffraction of the irradiated region showed a preferential orientation of the β-BaB₂O₄ crystallites on the glass surface.     

Lattice Parameter Local Determination for Trigonal,Hexagonal, and Tetragonal Crystal Systems Using Several Coplanar X-Ray Reflections

The method for determining the local lattice parameters using quasi-multiple X-ray diffraction (which was proposed and used only for crystals of the cubic system) has been expanded to measure the local crystal lattice parameters for the trigonal, hexagonal, and tetragonal systems. Local variations in the lattice parameters in the tetragonal paratellurite (TeO₂) and trigonal lanthanum-gallium tantalate (La₃Ga_5.5Ta_0.5O₁₄) crystals have been investigated. Reflections necessary for implementing the quasi-multiple X-ray diffraction scheme are found for these crystals. The pairs of reflections that were found were used to complete quasi-multiple X-ray diffraction schemes on a laboratory X-ray source and determine the variation in the lattice parameter a along the surface of these crystals. The relative measurement error was 4 x 10^-6 at a spatial resolution of 140 μm. The accuracy of the method and the parameters determining this characteristic were analyzed based on the results. The locality of the method and the limits of the spatial resolution of an X-ray diffraction measurement of lattice parameters are estimated.     

Effect of the defect structure character in calcite single crystals on X-ray diffraction

The paper presents experimental results for the dependence of the integral intensity of anomalous transmission T_i and Laue reflection R_i of X-rays on the structural quality of calcite single crystals as obtained by a two-crystal spectrometer. The relation between X-ray dynamic and kinetic scattering as a function of the sample thickness, densities of perfect dislocations in perfect CaCO₃ crystals and of atom-vacancy complexes in calcite crystals with „background”︁ has been found. Distortions due to perfect grown-in dislocations in calcite are shown to exceed those produced in crystals with „background”︁. The latter appear to be more X-ray transparent. A comparison with results of twin layer thickening experiments on CaCO₃ crystals of different qualities again points to the different nature of these distortions. In CaCO₃ crystals with the dislocation density of up to 2 × × 10³ cm⁻², the X-ray scattering, in our experiments, was consistent with the dynamic diffraction theory, while at higher dislocation densities, the X-ray scattering markedly differs from it.     

Incorporation of alkali impurities into single crystals of barium metaborate β-BaB2O4

The impurities in single crystals of the low-temperature β modification of BaB₂O₄ grown from flux in the BaB₂O₄-Na₂O system have been studied. The β-BaB₂O₄ compound was examined by X-ray powder diffraction analysis. The appearance of scattering centers in the crystals correlates with the high content of sodium whose critical concentration is about 0.021 wt %. The effective distribution coefficient of sodium is 2.5–4.4 × 10^?3. The conditions for constitutional supercooling at the crystallization front are considered.     

Influence of purity of NdF3 single crystals on their ionic conductivity

Single crystals of the NdF₃ superionic conductor have been grown by the Bridgman method from a melt in a helium atmosphere using a fluorinating PbF₂ agent. Commercial NdF₃ reagents of special purity grade, reagent grade, and pure grade are used. It is found that the ionic conductivity ?? of the crystals depends considerably on the purity grade of the starting substances: at 200°C ?? = 1.4 × 10^?, 3 × 10^?4, and 8 × 10^?4 S/cm for reagents of special purity grade, reagent grade, and pure grade, respectively.     

Study of microdefects in GaAs:Si single crystals grown by the vertical gradient freeze method

Microdefects in Si-doped GaAs single crystals grown by the vertical gradient freeze method have been studied with X-ray diffuse scattering. In the case of doping to majority carrier concentrations n ～ 1 × 10¹⁸ cm^?3, large microdefects with positive dilatation that accompany the initial stage of arsenic precipitation at high temperatures were observed. It is shown that GaAs samples heavily doped with silicon (n ～ 3 × 10¹⁸ cm^?3) contain large (several micrometers) interstitial microdefects, which can play the role of nucleation regions for new SiAs and SiAs₂ phases.     

Synthesis and characterization of Cr4+-doped CaO–GeO2–Li2O–B2O3(Al2O3) transparent glass-ceramics

Synthesis and devitrification behavior of Cr-doped CaO–GeO₂–Li₂O–B₂O₃(Al₂O₃) glasses have been studied. A range of glass compositions was found to yield transparent glass-ceramics after devitrification. The size of crystallites is below 1 μm. Glass-ceramic samples exhibit 1050–1600 nm broad-band emission with a maximum around 1260 nm, very similar to the emission of Cr⁴⁺:Ca₂GeO₄ bulk crystals. X-ray diffraction measurements indicate that the structure of crystallites exhibiting near infrared emission in glass-ceramics may be assigned to Cr⁴⁺:Ca₂GeO₄ with increased lattice parameters.     

Untersuchung epitaktischer Molybdänkarbidschichten an Molybdän-(110)-Oberflächen

The initial phases of high temperature (1700 K) carburization of (110) molybdenum surfaces by C₆H₆ at low pressure are examined with low energy electron diffraction (LEED) and emission electron microscopy. After an exposition of about 8 · 10⁻⁵ Torr · sec a (4 × 4) superstructure was observed by means of LEED, interpreted as coincidence lattices with the Mo₂C (10.0) plane parallel to the Mo (110) surface. Heating a sample without further exposition resulted in the appearance of a complex and of a (2 × 2) superstructures. After an exposition of about 2 … 9 · 10⁻³ Torr · sec needle shaped molybdenum carbide crystallites grew on the surface into the [111] and [111] directions. The orientation Mo₂C (10.1) parallel to Mo (110) was concluded from the LEED patterns.     

Defects and stresses in MBE-grown GaN and Al0.3Ga0.7N layers doped by silicon using silane

The electric and structural characteristics of silicon-doped GaN and Al_0.3Ga_0.7N layers grown by molecular beam epitaxy (MBE) using silane have been analyzed by the Hall effect, Raman spectroscopy, and high-resolution X-ray diffractometry. It is established that the electron concentration linearly increases up to n = 4 × 10²⁰ cm^?3 with an increase in the silane flow rate for GaN:Si, whereas the corresponding dependence for Al_0.3Ga_0.7N:Si is sublinear and the maximum electron concentration is found to be n = 4 × 10¹⁹ cm^?3. X-ray measurements of sample macrobending indicate a decrease in biaxial compressive stress with an increase in the electron concentration in both GaN:Si and Al_0.3Ga_0.7N:Si layers. The parameters of the dislocation structure, estimated from the measured broadenings of X-ray reflections, are analyzed.     

Direct measurement of isothermal compressibility of amorphous semiconductors

The isothermal compressibilities of the chalcogenide glasses Te₁₅Ge₃As₂, Te₁₅Ge₂As₃ and As₂Se₃ have been measured to 5 kbar by direct measurement of the change in sample length using a differential transformer placed inside the hydrostatic pressure vessel. Calibration data was obtained from the measurement of the compressibilities of alkali halide single crystals. For Te₁₅Ge₃As₂, the isothermal compressibility was found to be 6.00 × 10^?12 ? 0.7 × 10^?22P cm²/dyne; for Te₁₅Ge₂As₃, 5.58 × 10^?12 + 0.3 × 10^?22P cm²/dyne; for As₂Se₃, 6.30 × 10^?12 ? 0.8 × 10^?22P cm²/dyne where P is the pressure in dynes/cm². Errors are near 5%.     

On the Morphological Changes and Twinning of ZnS (Sphalerite) Crystallites under Hydrothermal Conditions

Morphological characteristics and twinning mechanism of ZnS crystals under hydrothermal conditions have been investigated in this paper. It was shown that under hydrothermal conditions the morphology of ZnS crystallites changes along the four‐fold axis directions, and the crystals are observed in a positive or negative tetrahedron, or in a combination of positive and negative tetrahedra depending on the growth conditions. The positive tetrahedral areas on the crystallites get larger with increase of the concentrations of OH^‐ and S^2‐ in solutions, whereas the twinned crystallites of ZnS taking an elliptic shape with (111) as composition plane are easily formed in weak basic solutions. It can be found that the morphologies of ZnS crystals are in accordance with the crystallization orientations of positive or negative coordination tetrahedra ([S‐Zn₄]⁶⁺, [Zn‐S₄]^6‐) in the crystal although, in some cases, the practical morphology could be greatly affected by growth conditions, and the twinning mechansim can be suggested based on the linkage of growth units of positive and negative coordination tetrahedra, which were formed in the solution. The present investigations further indicated that the crystal chemistry approach based on the linkage/incorporation of growth units previously proposed by us can be sucessfully applied to interpret the growth mechanisms of the crystals and to control a desirable morphology.     

Electrical and structural properties of annealed AsTe and AsTeI semiconducting glasses: surface and bulk effects

The effects of thermal annealing, above the glass transition temperature, on the electrical conductivity and structural properties of bulk As₅₀Te_50?xI_x (x = 0, 2, 5, 20) glasses have been studied. Upon devitrification, an electrically conductive layer is formed on the sample surface The conductivity of the surface layer is nearly temperature independent and has an activation energy of ≈10^?2 eV. X-ray diffraction experiments show that an fcc crystalline phase is associated with the surface layer. Helium ion backscattering experiments indicate that on a sample with a surface layer the average atomic composition does not change significantly with depth (probed to a depth of ≈2 μm) and is approximately the same as that of the as-prepared material. Results suggest that the conductive regions may be composed of a matrix of interconnected crystallites (fcc phase) with interdispersed non-crystalline material. As annealing progresses, crystallization extends throughout the bulk material with a corresponding increase in sample conductance. The electrical changes are considerably more pronounced in the x = 0 and x = 2 compositions. The degree of crystallization and the crystalline phases induced in the bulk material by annealing were determined using differential scanning calorimetry (DSC) and X-ray diffraction. These experiments show that following the anneal series (130, 155, 170, and 190°C; each for 5 in vacuum) the more conductive (x = 0 and 2) samples contain monoclinic As₂Te₃, whereas the fcc phase is observed in the x = 5 and 20 samples. Results also suggest that the first crystallization exotherm, T_X1, observed in the DSC data is associated with the formation of the fcc phase, whereas the second crystallization exotherm, T_X2, is associated with the conversion of the fcc phase to the high-temperature stable monoclinic As₂Te₃ phase. From all of these results a model is formulated for the amorphous-to-crystalline transformation and its effects on electrical transport in these semiconducting alloys.     

Thermoelectric properties of nanocrystalline solid solutions of bismuth and antimony chalcogenides

The samples structure of solid solutions of p- and n-type bismuth and antimony chalcogenides obtained in the form of rods 1.6 mm in diameter by hot extrusion have been studied. These rods are found to consists of nanocrystallites 8?C60 nm in size. In p-type samples, nanocrystallites aggregated into spherical crystallites 50?C100 nm in diameter, whereas nanocrystallites in n-type samples formed disoriented fibers with a transverse cross section of several ten nanometers to several micrometers and a length of a few micrometers to several dozen micrometers. The presence of nanocrystallites of this size in the sample structure reduces the thermal conductivity of bismuth and antimony chalcogenides by 10% but barely changes their resistivity and thermoelectric power. The thermoelectric efficiencies of the p- and n-type chalcogenides are 3.27 × 10^?3 and 2.78 × 10^?3 K^?1, respectively.     

Growth of MgF2 optical crystals and their ionic conductivity in the as-grown state and after partial pyrohydrolysis

MgF₂ single crystals have been grown from melt by the Bridgman technique in a fluorinating atmosphere. To control the presence of oxygen impurity, it was first suggested to measure the ionic conductivity in MgF₂ crystals by impedance spectroscopy. The characteristics of ionic conductivity of “as grown” (i.e., without thermal treatment) crystals and crystals obtained by commercial vacuum technology practically coincide: the volume conductivity σ_v = 1.4 × 10^?7 S/cm at 773 K and the ion-transport activation energy E _a = 1.40 ± 0.05 eV. Annealing MgF₂ crystals during electrophysical studies upon heating from 293 to 823 K in vacuum (residual pressure ～1 Pa) for 4 h led to their partial pyrohydrolisis. The influence of this thermal treatment of MgF₂ crystals on their optical transmission is studied in the wavelength range of 115–300 nm.     

Low-frequency dielectric properties of DMAAS crystals in the vicinity of 110 K

Dielectric properties in “pure” and partly deuterated DMAAS crystals have been studied within the frequency range 40^?2–10⁷ Hz in the vicinity of 110 K. It is established that the crystals possess relatively high conductivity, 10^?4–10^?7 Ω^?1 m^?1, which is explained by their crystal structure. The frequency dependence of the complex dielectric constant has two linear segments, which indicate the change in the charge-carriers motion in the vicinity of 110 K. It is revealed that at low temperatures, conductivity increases at higher frequencies.     

Low-temperature growth and doping of InP:S single crystals from melt-solution in microgravity

The effect of microgravity on the growth of bulk InP:S single crystals from a melt with an initial equilibrium composition (84 at % In, 16 at % P, and ～2.2 × 10¹⁸ at cm^?3 of S) on board the Foton-11 satellite was investigated. The growth of crystals on board the satellite and on Earth (a reference crystal) was carried out by the traveling heater method. The samples of the grown crystals were investigated by metallography, double-crystal X-ray diffractometry, single-and double-crystal X-ray topography, and secondary-ion mass spectrometry. It is shown that the mass transfer in the melt in microgravity is similar to the diffusion mode. Hence, the mass transfer in the melt results in the following: the formation of a nonstationary boundary layer, depleted in phosphorus; the constitutional supercooling at the crystallization front accompanied with the development of a cellular substructure in the early growth stage; and the hypothetical phase structurization of the transition layer with the formation of In-based associates (clusters), which were found in the grown crystals in the form of spherical defects 10–20 μm in diameter. The coefficients of sulfur distribution k₀ = 0.274 and k_eff = 0.43, the sulfur diffusivity in the melt D_S = 4.2 × 10^?7 cm²/s, and the effective thickness of the transition layer δ = 0.07 cm in terrestrial gravity are determined. The data obtained are necessary to develop a mathematical model of crystallization in zero gravity.     

Bulk single crystals of β-Ga2O3 and Ga-based spinels as ultra-wide bandgap transparent semiconducting oxides

In the course of development of transparent semiconducting oxides (TSOs) we compare the growth and basic physical properties bulk single crystals of ultra-wide bandgap (UWBG) TSOs, namely β-Ga₂O₃ and Ga-based spinels MgGa₂O₄, ZnGa₂O₄, and Zn_1-xMg_xGa₂O₄. High melting points of the materials of about 1800 -1930 °C and their thermal instability, including incongruent decomposition of Ga-based spinels, require additional tools to obtain large crystal volume of high structural quality that can be used for electronic and optoelectronic devices. Bulk β-Ga₂O₃ single crystals were grown by the Czochralski method with a diameter up to 2 inch, while the Ga-based spinel single crystals either by the Czochralski, Kyropoulos-like, or vertical gradient freeze / Bridgman methods with a volume of several to over a dozen cm³. The UWBG TSOs discussed here have optical bandgaps of about 4.6 - 5 eV and great transparency in the UV / visible spectrum. The materials can be obtained as electrical insulators, n-type semiconductors, or n-type degenerate semiconductors. The free electron concentration (n_e) of bulk β-Ga₂O₃ crystals can be tuned within three orders of magnitude 10¹⁶ - 10¹⁹ cm^?3 with a maximum Hall electron mobility (μ) of 160 cm²V^?1s^?1, that gradually decreases with n_e. In the case of the bulk Ga-based spinel crystals with no intentional doping, the maximum of n_e and μ increase with decreasing the Mg content in the compound and reach values of about 10²⁰ cm^?3 and about 100 cm²V^?1s^?1 (at n_e > 10¹⁹ cm^?3), respectively, for pure ZnGa₂O₄.     

The Nature of One-sided Intergrowths on (001) of Triclinic Centrosymmetric K2Cr2O7 (II)

SHUBNIKOV first observed in 1912 that centrosymmetric K₂Cr₂O₇ crystals (P1 ) exhibit a growth anomaly in highly supersaturated solutions. The (001) faces of these crystals are always smooth, the parallel opposite faces are always rough. Electron micrographs show these rough surfaces to be crystallites that have grown in stacks. In Part I of this publication, the crystallite orientation was determined using electron-induced diffraction methods. As a rule, these crystallites are parallel intergrowths. Only a small number of crystals near the surface of the host crystal are twinned according to [010]. Part II sought to ascertain the cause of these one-sided intergrowths. To determine the most favourable energetic arrangement of two intergrown individuals, interactions across the intergrowth face as a function of shifts parallel to (001) and to (001 ) were calculated, 64 theoretically possible cases were considered. Only in the [010]-twin and the seldom-occurring [11 0]-twin is an approximation of the 2/m symmetry of the macroscopic twin arrangements also found in the intergrowth structure, and this is the case only when the (001 ) faces of the host and guest crystal are intergrown.