Thermal expansion in CuAlTe2 compound

The lattice parameters a and c as well as the axial thermal expansion coefficients α ⟂ and α ∥ in the CuAlTe₂ chalcopyrite-type compound are determined as a function of temperature in the range from 80 to 650 K by a X-ray diffractometry technique. The data obtained are used to evaluate the axial ratio c/a, the tetragonal distortion δ = 2 — c/a, the interatomic distances for Cu Te and Al Te bonds and their temperature coefficients. It is found that the thermal expansion behaviour of CuAlTe₂ is similar to that of other CuB^IIIC^VI₂ compounds in having a relatively small expansivity along the c-axis and a large one in the perpendicular direction. When comparing the results for a series of the CuB^IIIC^VI₂ compounds (B Al, Ga, In; C S, Se, Te) it is shown that the correlations between the thermal expansion coefficients α ⟂, α ∥, α_m, dδ/dT and the tetragonal distortion δ, as well as the molar mass of the compound take place.     

An optical microscopy study of the biaxial ‐ calamitic nematic lyotropic phase transition

Lyotropic nematic textures are investigated, using optical microscopy, near the reentrant isotropic (I_RE) ‐ discotic nematic (N_D^‐) ‐ biaxial nematic (N_B) ‐ calamitic nematic (N_C⁺) ‐ isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol and D₂O. The N_B, intermediate phase between the two uniaxial nematic ones, is characterized by optical birefringence measurements. In this way, using a polarizing microscope and a colour CCD digital camera, the N_B – N_C⁺ phase transition is identified as well as the domain of the N_B phase and N_D^‐ – N_B transition point in accordance with optical birefringence data (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical absorption and photoelectric properties of Ga2In4S9 single crystals

Presented in this paper are the results obtained from the investigation of absorption edge and photoconductivity of layered Ga₂In₄S₉ single crystals as well as from the investigation of the photovoltaic effect in Pt—Ga₂In₄S₉ surface-barrier structures. We determined the magnitude of the direct band gap to be E_g = 2.72 eV and the position of the sensitizing centres to be ≈︁0.15 eV above the valence band top. In the regions of the photon energies smaller than E_g the absorption band edge is described by an exponential dependence with a temperature independent slope.     

The electrical and structural properties of InyGa1 ? yAs/InxAl1 ? xAs/InP quantum wells with different InAs content

In _x Al_{1 − x} As/In _y Ga_{1 − y} As/In _x Al_{1 − x} As/InP HEMT structures has been investigated with a change in the InAs molar fraction both in the quantum well and the buffer layer. The electrical parameters of the samples are measured at different temperatures. The structural parameters of the layers and the characteristics of the interfaces between them are determined by double-crystal X-ray diffraction. An increase in the Hall mobility and electron concentration, as well as in the structural quality of the samples, is observed alongside an increase in the InAs molar fraction in the quantum well. It is established that high electron mobility is retained at small (to 5%) mismatches between the buffer layer and substrate.     

On the decomposition behaviour of supersaturated Al-Zn-Mg alloys in dependence on heat treatment

The influence of the various heat treatments applied on the decomposition processes taking place in some Al-Zn-Mg alloys (varying in Zn as well as Mg content) are discussed based on the main assumptions that 1) the upper limit of the temperatur of homogeneous nucleation, T_hn, plays an important role on the kind of nucleation taking place at the aging temperature, T_a, and 2) vacancy-rich clusters, VRC, as well as overcritical GP zones can act as nucleation sites for η′;-particles at T_a > T_hn. The influence of the VRC is decreasing with rise of the Zn-content.     

On the influence of impurity additions and quenching temperature on the course of the microhardness (HV) during a two-step ageing treatment of an Al-2.0 at.%Zn-1.3 at.%Mg Alloy

A pure as well as the analogous technical alloy were directly quenched from T_q = 400 °C or 490 °C to T_a, preaged at T_a (ranging between 50 °C and 160 °C) various times (t_a), and subsequently postaged at R.T. up to t_post = 42 d. — Ageing the samples below the upper limit temperature of the homogeneous formation of G.P. zones (T_hn) no essential difference in the course of the HV-number attained after one- as well as two-step ageing in dependence on the addition of impurity atoms and T_q could be found. — Contrary to this behaviour after pre-ageing at T_a > T_hn and postageing at R.T. a variation of both T_q and the content of impurity atoms shows a considerable effect. The reason is the difference in the density of nucleation sites for heterogeneous formation of precipitates of the η′- and/or η-phase in dependence of T_q and impurity content.     

Bulk non-radiative recombination reduction in nGaAs due to an annealing under Ga-Al melt

It is shown by microcathodoluminescence and electron beam induced current investigations that efficiency of radiative recombination and the values of hole diffusion lengths in nGaAs substrate increase (up to L_p ∽ 5 μm) due to an annealing under Ga Al melt. The values of the annealing critical temperature exceeding of which leeds to bulk non-radiative recombination drop as well as optimal values of annealing temperature and duration are determined. In pAlGaAs–(p-n)GaAs solar cells based on annealed substrates the values of open circuit voltage U_oc = 1.16 – 1.18 V under 500-fold concentrated solar radiation as well as values of photocurrent comparable with the best results of solar cells with epitaxial nGaAs are achieved.     

Crystal and molecular structure of (20R)-19-nor-5β,14β-pregnane-3β,14,20-triol 3,20-diacetate

The title compound (20R)-19-nor-5β,14β-pregnane-3β,14,20-triol 3,20-diacetate crystallizes in the monoclinic space group P2₁ with 4 formula units C₂₄H₃₈O₅ in the unit cell. The lattice parameters are a = 14.324, b = 12.569, c = 12.685 Å, β = 90.21°. The crystal structure was determined by direct methods and refined by the least-squares procedure to the discrepancy factor R = 0.042. Bond length, bond angles, ring conformations, as well as the absolute configuration at C20 were determined. In the crystal the molecules are linked to infinite chains by two hydrogen bonds.     

GaP?AlxGa1−xP heterostructure edge coupling waveguides for hybrid integrated optic devices

In this paper we present the performance of GaP Al_xGa_1−xP heterostructure waveguides for integrated optic hybrid devices. The waveguide layer is graded with a parabolic refractive index profile for the light emission of a DH GaAs semiconductor laser. The waveguide heterostructure performed allows to couple at it edge crossection semiconductor laser or photodetector as well as fix or solder them on the same substrate on which the heterostructure layers were deposited. The heterostructure layers were grown by liquid phase epitaxy and the heterostructure waveguide was performed by means of etching technics specially developed for this purpose. Both, the growth procedure, as well as the etching technics are described. The results achieved are shown in curves and photographs, taken on a scanning microscope.     

Formation of a Banded Texture in Solutions of Liquid Crystalline Polymers: 1. Hydroxypropylcellulose in H2O

Abstract

The formation of a banded texture was investigated for sheared anisotropic solutions of hydroxypropylcellulose (HPC) in H₂O. A cone and plate rheometer made of highly polished glass allowed the exploration of the shear rate (γ) range between 0.1 and 150 sec^?1 and the study of the effect of shear time t_s. Texture, monitored with a video-camera, appeared after a time t_b following cessation of shear. t_b is usually a decreasing function of γ and t_s. In most cases a precursor banded texture precedes the banded texture, and t_b attains zero at high enough γ or t_s. However, when a particularly good orientation of the flowing solution is achieved, the precursor texture is not observed, t_b is finite, and the bands are particularly well formed. We speculate that bands are the normal relaxation mode for well oriented solution. For less oriented solutions flow instability may lead to bands even during flow.     

X-ray analysis of sideroxol from <Emphasis Type="Italic">Sideritis leptoclada</Emphasis>

Sideroxol (1), a kaurane diterpene which has the ent-7α,18-dihydroxy-15β,16β-epoxykaurane structure (MW = 320.47, C₂₀H₃₂O₃) was obtained from the acetone extract of Sideritis leptoclada plant as well as from some other Sideritis species. It crystallizes in the orthorhombic space group P2₁, 2₁, 2₁ with a = 10.967(3), b = 24.555(5), c = 6.372(4) Å, Dc = 1.240 g cm⁻³, Z = 4, and refines to R = 0.065 for 721 independent reflections. The skeleton consists of three fused six-membered rings and a five-membered ring with fused epoxide. The six membered rings exhibited slightly distorted chair conformation. In addition to sideroxol, two kaurane and five kaurene diterpenes were isolated from the hexane and acetone extracts of the studied plant.     

Crystal and molecular structure of p-Toluquinone-monoguanylhydrazone,C8H10N4O

The crystal structure of p-toluquinone-monoguanylhydrazone is orthorhombic, space group Pna2₁ with 4 formula units C₈H₁₀N₄O in the unit cell. The lattice constants are a = 7.589, b = 20.117, and c = 5.699 Å. The structure was refined to the conventional R = 0.05. The configuration and conformation of the molecule as well as the unusual hydrogen bridge system are discussed.     

Liquid Crystals: A Bibliography for 1975

Abstract

The static dielectric constants of biphenyl-4-p-n-alkoxybenzoates have been measured as functions of temperature. The dielectric anisotropy (δ?) for the successive homologues (hexyl to undecyl) shows an alternation similar to that exhibited by the nematic-isotropic transition temperature (T _NI). The dispersion of ?_∥ in the frequency range of 0.1-15 MHz has been studied for four successive members of biphenyl-4-p-n-alkoxybenzoates (heptyl to decyl derivatives) as well as of trans-p-n-alkoxy-α-methyl-p'-cyanophenyl cinnamates (eighth to eleventh members). For both series the relaxation frequency (f _R) and the frequency of dielectric isotropy (f _o) evaluated at a common relative temperature show an alternation similar to T _NI. For any given compound the Arrhenius plots for f _R and f _o give the same value of the activation energy (W). In the case of the biphenyl esters the vaue of W is the same for the seventh and eighth members, but increases for the higher members which exhibit a smectic phase at lower temperatures. On the contrary for cinnamic acid esters W is found to be the same for all homologues irrespective of whether the nematic phase is preceded by a smectic phase or not.     

Compressibility in the Tetragonal Phase of AgInS2 From the X-ray Diffraction Data under High Pressure

The lattice parameters a and c as well as the compressibilities for different directions in the AgInS₂ chalopyrite-type compound are determined as a function of pressure up to 5 GPa using a powder X-ray technique under high pressure. It is found that the compressibility is anisotropic, with compressibility along the tetragonal c-axis (𝓏_c) being smaller than that along the a-axis (𝓏_a). It is shown that as pressure rises the axial ratio c/a increases and as a result the tetragonal distortion δ = 2 - c/a reduces.     

Kinetic of potash alum crystallisation

Metastable zone width of potash alum in aqueous solutions has been measured as function of the cooling rate in absence as well as in presence of seed crystals. From these data, the effective nucleation rate was calculated. Growth rate of potash alum crystals has been measured in a fluidized bed. Overall growth rate coefficient, k_G, was then used for determination of the mass transfer coefficient, k_D, and the rate constant of the intergration step k_r. The dependence of k_D on hydrodynamic conditions is discussed.     

Dielectric characteristics of neodymium heptamolybdate crystals grown by gel encapsulation technique

Results obtained from the dielectric studies of neodymium heptamolybdate crystals grown in the system Nd(NO₃)₃ –MoO₃ –NH₄OH– HNO₃ —Na₂SiO₃ by gel encapsulation technique are presented. The variation of dielectric constant (ε′), dielectric loss (tanδ) and conductivity (σ) with frequency at different temperatures is studied. The dielectric constant of the material increases sharply, attains a peak value and then decreases rapidly, as material's temperature rises from room temperature to higher degrees. The temperature at which the peak value is attained is the transition temperature of the material. Dielectric loss (tan δ) follows almost a similar behaviour. The conductivity (σ) is also found to be temperature-and frequency-dependent. This sharp rise in ε′ is attributed to the contribution from space charge polarization. Theoretically obtained data using the expression ε′ = a₀ + a₁ T² and —In σ = a + b(T — T₀)² for T < T₀) fits very well with the experimentally obtained data indicating that space charge polarization increases as some power of temperature larger than one.     

Mechanical properties of single crystalline and glassy lithium triborate

Mechanical properties of LiB₃O₅ single crystal plates with different orientation as well as of glass with the same composition have been investigated. The nano‐ (H) and microhardness (H_M), the reduced Young's modulus (E_r) and the crack behaviour of the samples were studied. Both hardness and Young's modulus of glass appeared smaller in comparison to corresponding single crystal data (H ∼ 7 – 8 GPa, H_M ∼ 6 GPa, E_r ∼ 70 – 80 GPa for glass and H ∼ 10 – 15 GPa, H_M ∼ 6 –11 GPa, E_r ∼ 93 – 155 GPa for single crystal). H, E_r, and the plane of crack propagation proved orientation‐dependent. Cracks in the glass sample were not observed up to 0.49 N microindentation load, whereas for the single crystal the cracks appeared already at 0.098N. In single crystals the observed cleavage planes {211} and/or {412} are oriented nearly parallel to planes of B‐O rings. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stereochemistry of 1,2-oxaphospholanes. Part X. Synthesis,X-ray structure and NMR spectroscopy of (2S*,3R*,5R*)-2-methoxy-3,5-dimethyl-2-oxo-1,2-oxaphospholan-3-ol

The title compound (4a) was synthesized from a phosphite compound obtained from 4-hydroxy-2-pentanone and dimethylphosphorochloridite, after addition of an equivalent of water. Thecis-configuration of the P=O and C3-OH groups was established based on³¹P NMR chemical shifts. In chloroform, as well as in benzene solution, the most favorable conformations of the title compound are ₄ ⁵ T,⁴ E, and ₄ ⁵ T. The crystal and molecular structure of4a has been determined by single-crystal X-ray diffraction data, and refined toR=0.056. The five-membered ring has an envelope⁴ E (C₅) conformation, with the asymmetry parameter C ₅(4)=3.5°. Both the C5-CH₃ and the C3-CH₃ groups are equatorial. In the solid state, the molecules form dimers with hydrogen bonding between the hydroxyl groups and the phosphoryl oxygen atom of an adjacent molecule. A quantitative comparison of the structure of 4a with the structure of the related 2-mcthoxy-3,5,5-trimethyl-2-oxo-1,2-oxaphosphospholan-3-ol is given.     

Crystal structure of tetrabutylammonium nitroprusside dihydrate, [(C4H9)4N]2[Fe(CN)5NO]·2H2O, and vibrational spectra of ground and metastable excited states of the dihydrate and the anhydrate

The crystal structure of tetrabutylammonium nitroprusside dihydrate (TBANPDH, [(C₄H₉)₄N]₂[Fe(CN)₅NO]·2H₂O) was determined by single crystal X-ray diffraction (XRD) and the compound was also studied, as well as the anhydrous salt (TBANP), by room and low temperature Fourier Transform Infra-Red (FTIR) and room temperature Fourier Transform Near Infra-Red Raman (FTNIRR) spectroscopies. In addition to the ground states (GS), the metastable excited states (MSI and MSII) generated by laser excitation (488 nm) at low temperature were IR studied, both of the dihydrate and the anhydrate. TBANPDH crystallizes in the trigonal space group P3₂21, D ⁶ ₃, with a = b = 13.777(2), c = 22.039(2) Å, and Z = 3. The structure was solved employing 1273 independent XR reflections, with I > 2(I), by Patterson and Fourier methods and refined by full-matrix least-squares to R1 = 0.054.     

Extention of the “three partial rates” model to layer growth and composition of the ternary Si1–(x+y)GexBy system

The previously published “three-partial rates” model of Si_1–xGe_x layer growth (Kühne 1992) has been extended for describing Si_1–(x+y)Ge_xB_y ternary systems by taking additionally into account a partial rate of boron and a partial rate of boron-originated silicon deposition. Silicon deposition from a silane-germane-diborane-helium gas mixture at T = 500°C and P_total = 0.2 Torr is clearly enhanced by the presence of germane though to a low degree only, but not by diborane. Germanium partial rate as well as boron partial rate depends linearly on the partial pressure of the respective source gas. For illustration the data published by Murase have been used.