Effect of an External Magnetic Field on Polytypism of CdI2 Crystals Grown from Solutions

The effect of growth conditions on the polytypic structure of crystals of CdI₂ was analyzed for crystallization from solutions. Three solvents were used: H₂O, 3 H₂O + 1 C₂H₅OH and 1 H₂O + 1.C₂H₅OH. Crystals were grown at two temperatures: 5 and 25°C with low and high growth rates; an external magnetic field of about 0.25 tesla was used. The effect of the above three parameters on the formation of the basic polytypes 2H and 4H and on the ordering of faults in disordered structures and polytype cells was studied by X-ray analysis of crystal surfaces. Some distinct relations between the polytypic structure of crystals of CdI₂ and the magnetic field were found.     

Effect of an External Electric Field on Polytypic Growth of CdI2 Crystals from Solutions

The effect of an external steady electric field on polytypic growth of CdI₂ crystals was investigated. Crystals were grown at four temperatures: 5, 25, 35, and 50°C with the use of four solvents: H₂O, C₂H₅OH, 3 H₂O + 1 C₂H₅OH and 1 H₂O + 1 C₂H₅OH. The electric field of the intensity 500 to 2500 V/cm was oriented in two directions: perpendicular or parallel to the c-axis of hexagonal plates of CdI₂ crystals. The analysis of the effect of the field on polytypism of CdI₂ was based on the analysis of the structure of about 800 crystals grown in the presence of electric field and of about 1000 crystals grown in neutral conditions. It was established that the weak electric field used for crystallization of CdI₂ does not influence noticeably the relative stability of basic polytypes (2H, 4H) and of complex polytypic structures (ordered and disordered). The electric field may influence the structure of CdI₂ crystals in two ways: (i) the polytypes formed in the presence of the field have on average two times larger cells than the polytypes formed in neutral conditions, (ii) the polytypes formed in neutral conditions have usually hexagonal cells; only 6 to 30% of polytypes are of rhombohedral type. When the electric field was used the percentage of rhombohedral polytypes increased to about 65%.     

Impurity induced structural phase transformations in melt grown single crystals of lead iodide

Lead iodide is a wide‐band gap and highly resistive semiconductor considered to be a promising room temperature nuclear detector. The phenomenon of polytypism is posing interesting problems of phase transformations among its polytypic modifications and formation of polytypic admixture during growth due to native impurities. Transformations have also been observed even when the material is stored for few months that causes deterioration in functioning of the PbI₂ devices. Lead iodide has been purified and single crystals were grown using zone‐refining system. The observed phase transformations during growth and storage have been explained in the light of distortion of [PbI₆]^4‐ octahedron due to impurities present in the material and the known crystal structures of PbI₂. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Working Model for Explanation of Polytype formation Based on Superlattice Ordering of Point Defects in Close Packing Structures

The model is an attempt to give an explanation of polytype orderings in the crystals. Basic assumptions of the model are: the point defects play the important part in the polytype formation; these defects influence the configuration (hexagonal or cubic) of individual layers. The defects have a tendency to ordering themselves into superlattices. It acts selectively on particular stacking of the layer sequences. The results of model operation are shown in examples of experimental data of polytypic materials; they are SiC and A^IIB_VI compounds.     

Experimenteller Nachweis der Temperaturfelder beim Schaltvorgang in Cu2O und Zeitlupenstudie des thermischen Durchschlages

The temperatures before and during the breakdown (switching) in planar Ag Cu₂O Ag structures were determined experimentally. They were measured by means of liquid crystals and thermo-chromatic cryons as temperature indicators as well as measuring the intensity distribution of the emitted IR radiation. Structural changes during electrical switching were observed using a slow-motion camera. The formation of the switching channel and the movement of the liquid phase in it were observed. The results show that the electrical switching is due to a thermal mechanism.     

Influence of TiO2 and ZnO nanoparticles on orientational order parameter of LC compounds – An optical study

In the present paper ZnO and TiO₂ nano particles are dispersed in Cyano-biphenyl liquid crystalline compounds (n?=?7, 12). The thermal polarizing microscopy and differential scanning calorimetry techniques are employed to measure the transition temperatures. The Nematic transition temperatures are decreased by 2.34?°C and 1.53?°C in Heptylbiphenyl, 1.07?°C and 1.12?°C in dodecylbiphenyl compounds due to the dispersion of nanoparticles. The refractive indices and Newton’s rings methods are exploited to measure birefringence at different temperature in nematic phase. The orientational order parameters are estimated by finding the birefringence in perfect order. The orientational order parameter is increased in nanoparticles dispersed liquid crystals when compare to pure liquid crystalline compounds.     

Morphological and X-ray study of SiC crystals grown from gas phase

SiC crystals obtained from the gas phase were investigated in order to characterize their morphology and polytypic modifications. Specific X-ray patterns showed the occurrence of one-dimensional disorder in some of α-SiC crystals.     

Some observations on melt-grown cadmium iodide crystals

Single crystals of richly polytypic compound cadmium iodide have been grown using a zone-refining system. The frequent appearance of cracks in the grown crystals has been attributed to the thermal stresses and the presence of impurities. Structure of all the crystals has been found to be of the type 4H.     

Preparation and properties of ferroelastic/ferroelectric polytypes of antimony(III) oxide iodine,Sb5O7I

Antimony oxide iodide, Sb₅O₇I (?7Sb₂O₃.SbI₃), is monoclinic and exists in at least 8 polytypic modifications. Crystals of these (transparent, pseudohexagonal prisms up to 15 × 10 × 5 mm³) were grown by vacuum sublimation and identified by optical and X-ray methods. Depending on the stacking sequence of characteristic slabs of antimony and oxygen in the (monoclinic) b-direction, centric and acentric polytypes result. The former show pure ferroelastic, the latter combined ferroelastic + ferroelectric behaviour. All polytypes transform reversibly into hexagonal, nonferroic phases at characteristic Curie temperatures between 165 and 208°C. The crystals can assume one of the three stable orientation states which may be mutually switched into each other by successive application of comprehensive stress (of the order of 10⁶ N/m²) on the three pairs of prism faces. In the ferroelectric polytypes the spontaneous polarization (about 5 × 10^-3 C/m²) is not reversible but only reorientable by ± 120° by coercive fields in the order of 2–5 kV/cm. Ferroelectricity and ferroelasticity are fully coupled.     

High temperature synthesis and crystal structure of new representatives of the huntite family

Single crystals of REE, Al borates, new and known representatives of huntite family are obtained by annealing of REEBO₃ on Al₂O₃ surface at 1100°C. Phase identification for REE=Pr, Eu, Tb, Tm, Ho, Yb has been carried out using unit cell parameter determination on single crystals. Crystal structures of C 2/c (PrAl₃(BO₃)₄) and C 2 (EuAl₃(BO₃)₄) ‐ two polytypic modifications, previously determined for other REE's have been studied. Formation of one or another polytype in similar thermodynamic conditions is probably dependent on electronic structure of rare earth ion. Crystal field stabilization energy in crystal fields of different symmetry is a possible factor of polytype stability (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of synthesis and postgrowth treatment conditions on the structural homogeneity of YBa2Cu3Ox single crystals

The effect of synthesis and postgrowth treatment (by annealing at various temperatures and partial oxygen pressures) conditions on structural homogeneity of YBa₂Cu₃O_x single crystals has been studied. The obtaining of single crystals with both high oxygen content and high critical temperatures T_c has been optimized. The wide range of postgrowth treatment conditions proposed allows to receive single crystals with given oxygen content from 6.2 to 7.0.     

Mesomorphism in chloro substituted isomeric series including chalconyl central bridge

A novel homologous series of thermotropic liquid crystals α-4-(4′-n-alkoxy benzoyloxy phenyl) β-2″-chloro benzoyl ethylenes have been synthesized and studied with a view to understand and establish the effect of molecular structure on liquid crystal (LC) properties with reference to molecular flexibility in isomeric series with differing positional status of same functional group. Novel homologues series consist of 12 homologues C₁ to C_16. C₁, C₂, and C₃ homologues are nonliquid crystals (NLC) and rest of the homologues are liquid crystals. C₁₀ to C₁₆ homologues are enantiotropically smectogenic plus nematogenic and C₄ to C₈ homologues are enantiotropic nematic. The texture of nematogenic derivatives is threaded or schlieren and that of the smectic mesophase are focal conic of the type smectic A or C. Analytical, thermal and spectral data supported molecular structures of novel homologues. Transition temperatures as determine by a hot stage polarizing optical microscopy (POM) were plotted against number of carbon atom present in n-alkyl chain ‘R’ of left n-alkoxy (-OR) group and the phase transition curves Cr–I/M, Sm–N, N–I were obtained on linking like or related points. The odd–even effect is observed for the N–I transition curve and thus transition curves behaved in normal manner. The even-membered nematic transition curve occupied higher position than the odd-membered transition curve. Present series is predominantly nematogenic and partly smectogenic with middle-ordered melting type.     

Growth and etching of anhydrous diglycine sulfate single crystals

Single crystals of anhydrous diglycine sulfate are grown at constant temperature by evaporation method. The crystals are grown at various temperatures and their growth rate is determined. A chemical etchant for dislocation studies is reported. The perfection of the crystals grown at various temperatures has been studied using etching technique. It is found that the crystals contained inclusions of the mother liquid. The crystals grown below 28° are almost free of inclusions and the dislocation density is also low.     

Study of Molecular Structure,Mesomorphism and Their Relation with Molecular Rigidity-Flexibility

Entirely smectogenic novel chalconyl homologue derivatives of thermotropic liquid crystals (LC) have been synthesized with a view to understanding and establishing the effects of molecular structure on liquid crystal properties and the degree of liquid crystallinity. The homologous series consists of 12 (C₁ to C₁₆) homologues with absence of nematogenic character throughout the present series. The textures of smectic phase are fan shaped or batonnet type of smectic-A or of smectic-C. Phase transition temperatures and thermotropic data were determined by an optical polarizing microscope equipped with a heating stage. The smectic–isotropic (Sm-I) phase transition curve and the Cr-Sm transition curve behave in normal manner in the phase diagram. Analytical and spectral data confirm molecular structures of novel homologues. The average thermal stability for smectic is 139.42°C and the mesogenic phase length ranges from 3.5°C to 79.6°C. Mesogenic properties are compared with the structurally similar known series.     

Identifying nucleation temperatures for lysozyme via differential scanning calorimetry

Production of diffraction-quality protein crystals mandates nucleation of a moderate number of crystals. Excessive nucleation can lead to many small crystals of poor morphology, making them useless for structural determination via X-ray crystallography. In this work, differential scanning calorimetry (DSC) and light scattering are utilized to delineate the liquid–liquid binodal in lysozyme solutions at concentrations typically used for crystal growth (10–50 mg/ml). Such cloud point temperatures are determined over a range of protein concentrations and compared to visually observed nucleation temperatures. In all cases, nucleation occurs at temperatures above the cloud point temperature. Thus, measurement of the cloud point temperature gives the crystal grower a bound on the protein crystal growth problem. Nucleation is reproducibly induced within 2°C of the cloud point temperature; while prior work indicates that growth, when carried out near the saturation temperature, yields high quality crystals. The ease of measuring the cloud point via turbidity measurements allows for the rational selection of an initial temperature to induce protein nucleation.     

Effects of Nematic Compounds with High Clearing Points on the Multiplexing Performance of Twisted Nematic Displays

The addition of nematic compounds with high clearing points (N_h) to multiplexable nematic liquid crystals decreases the temperature dependence of the threshold voltage of twisted nematic displays. This effect depends on the dielectric anisotropy of the nematic liquid crystals containing N_h, as well as on the increase in T _c. In the case of 4-n-butylphenyl 4'-(4”-butyibenzoyloxy) benzoate, the low threshold voltage was exceptionally compatible with a small temperature dependence of threshold voltage, and this result was attributed to the large temperature dependence of the dielectric anisotropy in comparison with that of nematic liquid crystals containing other N_h's.     

Asymmetrical Azobenzene Liquid Crystals with High Birefringence Prepared via Click Chemistry

Asymmetrical azobenzene liquid crystals with high birefringence were prepared via the click chemistry method. The chemical structures of the synthetic intermediates and compounds were confirmed by FT-IR and ¹H-NMR. The liquid crystal behavior, including transition temperatures and phase sequences, were investigated by DSC and POM. The effect of the alkyl substituents on liquid crystal behavior is discussed in detail. Then, the birefringence values of the liquid crystals were measured using polarizing light interferometry method. Moreover, the characteristics of selective reflection, which was associated with the value of birefringence, were studied.     

沿[1-100]方向升华法生长6H-SiC单晶

本文采用升华法沿着垂直于c轴方向的[1-100]方向生长6H-SiC单晶.利用光学显微镜对晶体表面及腐蚀后的晶片进行观察,发现沿[1-100]方向生长出的单晶与传统方法沿[0001]方向生长单晶有很多的不同之处,多型对于籽晶的继承性非常强,但是在生长过程中多型夹杂不会发生,该方法生长的晶体中没有发现螺位错(微管)缺陷.     

Zur Züchtung von CaF2-UND CaF2(U)-einkristallen aus der dampfphase

Monocrystals of pure and uranium-doped CaF₂ have been grown from the vapour phase in graphite crucibles using a vertical moving gradient method with seed-selection. The growth technique and the properties of the vapour phase grown crystals are described. Evaporation temperatures: 1450–1550 °C, δT≌ 65 °C, growth temperatures: ca. 1300–1400 °C. Dopant: UF₄. Size of the crystals up to 18 mm in diameter and 50 mm in length. In accordance to the low colour center formation by X-raying the pure CaF₂ crystals contain only a small number of lattice defects. The uranium content and distribution in the doped crystals depends strongly on vaporization temperature and inner shape of the crucibles, the valence of the uranium in the CaF₂(U) crystals is mainly determined by the fluorine vapour pressure in the growth system. Tetravalent U can be incorporated in the presence of PbF₂ and trivalent U under high vacuum.