On the interrelation between relaxation and hydrodynamic phenomena in nematic liquid crystals

The theoretical prediction of the interrelation between the relaxation processes and hydrodynamic phenomena observed in nematic liquid crystals (NLCs) under binary acoustic action at MHz frequencies is experimentally substantiated. The existence of a new (relaxation) mechanism of streaming generation in thin layers of the mesophase is proven by the results of measurements of the velocity of steady-state acoustic streaming in the NLC in a planar capillary with homeotropic boundary conditions induced by the simultaneous action of coherent compressional and shear waves. A model that takes into account not only the classical convective contribution to the streaming formation but also the nonlinear relaxation one is tested for the first time. It is shown that only this model describes the aforementioned phenomenon consistently with the experimental data.     

On the mechanism of chemical polishing of GaAs crystals

In order to establish reproducible conditions for chemical lap polishing of GaAs with NaOCl solutions, containing also OH⁻- and CO -ions, the mechanism of dissolution was investigated using the rotating disk arrangement. In solutions with excess OCl⁻ concentrations likewise diffusion and a reaction of the first order with respect to OH⁻ and CO, complexing agents for Ga-ions, control the dissolution rate, and the surfaces of the wafers are polished. OH⁻ and CO₃^2– concentrations in excess with respect to OCl⁻ lead to rate determining OCl⁻ diffusion and to irregularly polished or film covered surfaces. Diffusion constants for OH⁻, CO and OCl⁻ are given.     

Enhancement of thermoelectric efficiency in vapor deposited Sb2Te3 and Sb1.8In0.2Te3 crystals

Pure and indium doped antimony telluride (Sb₂Te₃) crystals find applications in high performance room temperature thermoelectric devices. Owing to the meagre physical properties exhibited on the cleavage faces of melt grown samples, an attempt was made to explore the thermoelectric parameters of p‐type crystals grown by the physical vapor deposition (PVD) method. The crystal structure of the grown platelets (9 mm× 8 mm× 2 mm) was identified as rhombohedral by x‐ray powder diffraction method. The energy dispersive analysis confirmed the elemental composition of the crystals. The electron microscopic and scanning probe image studies revealed that the crystals were grown by layer growth mechanism with low surface roughness. At room temperature (300 K), the values of Seebeck coefficient S (⊥ c) and power factor were observed to be higher for Sb_1.8In_0.2Te₃ crystals (155 μVK⁻¹, 2.669 × 10⁻³ W/mK²) than those of pure ones. Upon doping, the thermal conductivity κ (⊥ c) was decreased by 37.14% and thus thermoelectric efficiency was improved. The increased figure of merit, Z = 1.23 × 10⁻³ K⁻¹ for vapour grown Sb_1.8In_0.2Te₃ platelets indicates that it could be used as a potential thermoelectric candidate.     

Growth and morphology of the single crystals of thermoelectric oxide material NaxCoO2

We have grown single crystals of recently discovered thermoelectric oxide material Na_xCoO₂ using NaCl flux. Crystals of sizes upto 1.5 x 1.5 x 1.5 mm³ having different morphological habits were reproducibly grown. The atomic force microscopic studies show that along c‐axis crystals grow via 2D layer‐by‐layer mechanism. The X‐ray diffraction analyses show that grown crystals are rich in Na content as compared to the starting charge indicating that NaCl flux also acts as a source of Na. The resistivity of the crystals exhibited a linear temperature dependence in the region between 30 and 300 K. © 2004 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.     

On the growth mechanism of polycrystalline snow crystals with a specific grain boundary

Observed frequently in a natural snowfall at temperatures lower than -20°C, the crossed-plate crystal is an assemblage of two to four thin basal plates, each plate extending along a CSL grain boundary with a relation of 70.3°/[11$\text{2}$0]. A possible growth mechanism is proposed to account for the characteristics growth in the direction of the axis [11$\text{2}$0]. If two or more grain boundary dislocation (GBD) structures are contained at a CSL grain boundary, growth layers will overflow alternately on the (10$\text{1}$0) planes of each component crystal at the intersection of these planes. Thus, the distinctive structure of the CSL grain boundary, like the feature of a screw dislocation step, will induce the preferential growth in this direction.     

On the mechanism of trace-element uptake during the hydrothermal growth of sulfide mineral crystals

The stable species of cadmium and mercury are formed in iron and lead sulfides at elevated temperatures and pressures under hydrothermal conditions. Their formation proceeds through different mechanisms. The most general mechanism involves the uptake of trace elements in the form of structural species due to isomorphous exchange. The results obtained for cadmium in pyrrhotite indicate that structural defects substantially affect the incorporation of impurities into the crystal structure. It is found that trace elements are accumulated on the surface in the absence of defects and are incorporated into the crystal bulk in the form of structural impurities in the presence of defects. Moreover, at elevated temperatures and pressures, trace elements can form their own (surface nonautonomous) phases. These phases are formed not only at trace-element concentrations close to saturation (Cd in Fe_1.0S) but also under conditions far from saturation as a result of the interaction of trace elements with the oxidized surface of the mineral (the cadmium uptake by galena with the formation of either a sulfate surface phase or a sulfate-chloride surface phase). An important mechanism of trace-element uptake by sulfides during the hydrothermal growth of crystals is associated with external and internal adsorption of impurities by defects. This adsorption manifests itself both in an increase in the content of sorbed mercury in galena at a high sulfur activity due to the interaction of mercury with lead vacancies and in the formation of dislocation cadmium species, which results in an increase in the coefficient of cadmium distribution in the crystal-solution system.     

Specific features of the optical spectra in Tl2In2S3Se layered single crystals

The optical properties of Tl₂In₂S₃Se layered single crystals have been studied by means of transmission and reflection measurements in the wavelength range of 450‐1100 nm. The analysis of the absorption data revealed the presence of both optical indirect and direct transitions with energy band gaps of 2.16 and 2.42 eV, respectively. The observed coexistence of indirect and direct energy band gaps is related to the anisotropy of the Tl‐containing layered crystals. The dispersion of the refractive index is discussed in terms of the Wemple‐DiDomenico single‐effective‐oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero‐frequency refractive index were found to be 4.78 eV, 43.58 eV, 13.18 × 10¹³ m^–2 and 3.18, respectively. From X‐ray powder diffraction study, the parameters of monoclinic unit cell were determined. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of annealing on the conduction mechanism in amorphous silicon

This paper reports the effect of annealing on conduction mechanisms in amorphous silicon films. High field transport is mainly by the Poole-Frenkel mechanism in unannealed films of amorphous silicon. As they are annealed, the conduction meachnism changes from Poole-Frenkel to space charge conduction. The films are fully annealed at 120°C for 15 h and any more annealing for longer periods does not change the I–V characteristics. From the measured values of threshold voltage at which space charge conduction sets in, the density of localized states around the Fermi level is calculated to be about 10¹⁷–10¹⁸/eV · cm³ which agrees very well with our results from other experimental techniques.     

On the incorporation mechanism and distribution mode of impurities in potassium acid phthalate crystals grown from aqueous solutions

Mechanism of impurity structure formation in crystals grown from aqueous solutions has been studied on the example of potassium acid phtalate (abbreviated hereafter as KAP) single crystals. Gold decoration technique at an electronmicroscopic scale has been applied to the study of the distribution of uncontrolled impurities on KAP cleavage face (010) after 10, 20 and 30 days of growth, taking into consideration different growth rates in 〈001〉 and 〈001 〈 directions. A technique for visualization of impurities in water, based on the adsorption of these impurities by the surface of amorphous film of nitrocellulose (parlodion) and the vacuum decoration with gold of these impurities, has been developed. Differences in the impurity structure of KAP regions located in 〈001〉 and 〈001〉 directions from the seed have been established. In 〈001〉 direction after 20 days of growth impurity assemblies 0.1—0.4 μm in size are revealed, and in 〈001〉 direction heterogeneous impurity structure is revealed only after 30 days of growth. The real (impurity) structure of KAP outside impurity assemblies is quite homogeneous and is the same throughout the whole crystal volume, the impurities incorporating mainly into complex active centres. From comparison of the changes in time of the impurity structure of water used for crystallization solutions and the impurity structure of KAP crystals a conclusion is made that the impurity structure of crystals is “programmed” in the impurity structure of crystallization solutions which regularly changes with time, i. e. impurities from different kinds of assemblies which are selectively adsorbed by the growing crystal faces. The role of the adjacent to the growing face interfacial layers which control the growth rate and have a complex impurity structure is stressed.     

Study of the growth mechanism in the ADP-type anion-doped crystals

A simultaneous simulation of the growth process in the A(K)DP-type anion-doped crystal and growing of this crystal from aqueous solution under controlled conditions has made it possible to analyse the mechanism of impurity influence on the growth rate of various crystal faces. The anisotropy of the face growth rate and the capture coefficient of various impurities by the faces have been determined by the difference in dissolution heats and in sizes of replacing ions. The increase in energy change, when the ion in a solution enters a crystal, causes an increase in the growth rate and the capture of impurities by the face with the largest change. Breaking of one of the joined faces and increased growth of another one resulted in forming of the new faces, i.e. in changing crystal morphology.     

Elastic properties of the dithionates Na2S2O6 · 2 H2O,K2S2O6, Rb2S2O6, CaS2O6 · 4H2O and SrS2O6 · 4 H2O

Single crystals of the title compounds having optical quality and dimensions of several cm were grown from aqueous solutions. The elastic and thermoelastic constants were determined from ultrasonic resonance frequencies of thick plates. The true point symmetry of K₂S₂O₆ and Rb₂S₂O₆, which is screened by a hexagonal hypermorphy, could be clearly revealed to be trigonal (32) by the existence of the elastic constant c₁₄. In the case of CaS₂O₆ · 4H₂O and SrS₂O₆ · 4H₂O the constant c₁₄ of the specimens appeared too small to confirm the trigonal symmetry group required from electrooptic and non-linear optic effects unambiguously. The isotypy of K₂S₂O₂ and Rb₂S₂O₆ as well as that of CaS₂O₆ · 4H₂O and SrS₂O₆ · 4H₂O is confirmed by their elastic behaviour. The mean elastic stiffness of dithionates is closely related to that of the corresponding sulphates. In the vicinity of the second-order phase transition of K₂S₂O₆ near 235 K weak anomalies of the temperature derivatives of the longitudinal elastic stiffnesses are observed.     

On the influence of nonstoichiometry on luminescent properties of CdGa2S4 single crystals

Results of a study of photoluminescence and thermoluminescence of cadmium thiogallate single crystals grown from the melt are presented. Effect of nonstoichiometry on the properties of samples is studied. Some assumptions about the nature of centres in CdGa₂S₄ compound are made.     

Crystal growth and defects in Ga3PO7 crystals

Crystals with a non‐centrosymmetric structure are of great interest owing to their properties such as ferroelectricity, piezoelectricity, dielectric behavior and optical properties. In this letter, Ga₃PO₇ crystals are grown by the top‐seeded solution growth (TSSG) method from a Li₂O‐3MoO₃ flux. It crystallizes in a non‐centrosymmetric trigonal crystal system with space group R3m within point group 3m. The growth defects are investigated by means of chemical etching method. The results reveal hot concentrated phosphoric acid to be a good etchant for Ga₃PO₇. The main defects are cracks, inclusions, dislocations and twin. In the meantime, the effective measures for reducing the defects are proposed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of various external factors and pretransitional phenomena on structural transformations in cholesteric liquid crystals

The publications on low-molecular thermotropic liquid crystals with cholesteric structures have been reviewed. The effect of an applied electric field on cholesteric structures is studied. Bistability of the cholesteric-nematic transition, electric field-induced color textures of cholesteric mixtures, and electro-optics of amorphous cholesteric structures are considered as well as pretransitional (blue and TGB) phases, discotic cholesterics, PDLC films, flexoelectric electro-optics, and photostimulated switching in cholesterics.     

On the void distribution and size in shaped sapphire crystals

Ribbon and rod sapphire pulling has been performed in three different crystal growth equipments in order to study the effect of the installation, of the atmosphere, of the die shape, of the feed material and of the pulling rate on the distribution, number and diameter of the characteristic voids (micro‐bubbles) in the crystals. The location of the bubbles in the crystals depends on the die geometry; however, in most cases they are essentially located close to the crystal periphery and then can be efficiently removed by lapping. After statistical analysis of the results, it is demonstrated that the number of gas moles incorporated in the crystals, inside the voids, is totally independent of any growth parameter. It is also shown that the bubble diameter depends only on the pulling rate. Consequently, for a given pulling rate, the number of bubbles auto‐adjusts in order to satisfy the constant molar gas incorporation. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Determination of the concentration of conduction electrons in Y3Fe5O12 garnet crystals

Ferrites (garnets) are a model object of study and a promising material to be used in magneto-optical devices for data recording and processing and IR modulators. Due to the narrow ferromagnetic resonance line, optical transparency, and high elastic Q factor of ferrite single crystals, they are promising for solid-state microwave, optoelectronic, and computation devices. To ensure the optimal application of ferromagnetic materials, it is necessary to complete the following important task: develop a certification for samples with allowance for the degree of their imperfection caused by the deviation of crystals with garnet structure from stoichiometry, the competitive incorporation of process and dominant impurities, oxygen vacancies, etc.