The theoretical growth form of potassium nitrate from an aqueous solution

The theoretical growth form the potassium nitrate, obtained from a PBC analysis, is compared to the growth forms obtained from aqueous solutions. The discrepancies are explained satisfactorily in a semi-quantitative way by taking the solvent interaction into account. It was assumed that the desolvation of the crystal surface determines the growth rate. The solvent interaction was obtained from molecular mechanics calculations on water molecules at the solvent accessible surfaces of the crystal faces. It was observed that the water molecules orient preferentially with their dipole moments parallel to the surface to form a “skin”, kept together by hydrogen bonds.     

Formation of crystal structure of cyanovinylbenzene derivatives: X-ray structure investigation of ethyl esters of 3,4-dimethoxy-, 3,4,5-trimethoxy-, and 4-fluorobenzylidenecyanoacetic acids

Ethyl esters of 3,4-dimethoxy-, 3,4,5-trimethoxy-, and 4-fluorobenzylidenecyanoacetic acids are investigated by X-ray diffraction analysis with the aim of elucidating the factors responsible for the formation of crystal structures of cyanovinylbenzene derivatives. The crystal data for other known derivatives of this series are analyzed. It is revealed that, in the absence of strong hydrogen bonding, the main structure-forming function in crystals is fulfilled by intermolecular contacts of two types, namely, staking contacts of parallel and antiparallel molecules. The staking contacts of antiparallel molecules are most frequently realized. It is assumed that their presence in the structure determines the prevailing formation of centrosymmetric packings in the compounds under consideration.     

Nematic-isotropic phase transition in polar liquid crystals. 1. Statistical theory

The theory of transition from the nematic to the isotropic phase for liquid crystals in the system of rodlike particles with large longitudinal dipoles has been developed with due regard for the equilibrium between monomers and antiparallel molecular pairs—dimers. The order parameters of monomers and dimers are determined as well as the dimer fraction. It is shown that, in accordance with the results obtained earlier, for low values of dipole moments, the temperature of the nematic-isotropic phase transition increases with an increase of the dipole moment. However, for large dipoles, the transition temperature starts decreasing with an increase of the dipole moment because of higher dimer concentration. This provides the interpretation of the recent computer simulation, which showed a destabilization of the nematic phase in the system of rigid rods with pronounced central dipoles. The temperature dependence of the dimer fraction is also studied. The qualitative relation between the sign of the jump in the dimer fraction at the transition point and the effect of dimerization on the transition temperature are established.     

Dependence of Mesogen Molecules Interaction Energy on their Mutual Orientation

For interaction energy of mesogenic model rod-like molecules an analytical expression is obtained. Interaction energy of dipole moments interaction is much less than the total interaction energy. The empirical Rapini-Papoulard potential for the free surface energy of liquid crystals corresponds well with a limiting case of the expression obtained for small orientation angles.     

Effect of Cu Ions on the Dipole Moments and Polarization of Strontium Barium Niobate Single Crystals

The crystal growth, the ferroelectric hysteresis loops, and the infrared reflectivity spectra of coppermodified strontium barium niobate (SBN) single crystals are reported. Compared with the undoped SBN crystals, the copper-modified crystals have stable hysteresis loops, the spontaneous polarization of which is about 0.30 C/m², but that of the undoped SBN crystals is only about 0.12 C/m². Their coercive field strength is all about 340 V/mm. The experimental results of the infrared spectra show that the infrared reflectivities vary as the orientations of the dipole moments owing to the copper-doping. The c-axis becomes the most stable orientation of the dipole moments, and the polarization will be locked and won't recede if the copper-modified crystals are polarized into monodomains.     

Optical and dielectric properties of an isothiocyanato liquid crystal compound and comparison of its behavior with that of a homologous member

The properties of an isothiocyanato liquid crystal compound (code name S1) has been studied with a view to compare its behavior with that of another member of its homologous series (code name S5). Optical properties, such as refractive index, polarisability, and their anisotropies, have been evaluated with varying temperature and the orientational order experimentally determined from the study of polarisability anisotropy and compared with theoretically computed values. Dielectric studies have been conducted to determine not only the dielectric anisotropy but also the effective molecular dipole moment and its temperature dependence. The temperature variation of the angle of inclination (β) of molecular dipole moment with the director direction has also been determined and compared with that of S5. In order to determine the nature of molecular association in the mesophase, the molecular correlation factor (g) has been estimated from the measurements of dipole moments of compound in solution and compared with the values obtained for S5.     

Oberflächenmorphologie und Gleitverhalten trikliner Einkristalle von TCNQ-Komplexsalzen

The electron microskopic investigation of the most important crystal faces of the TCNQ-complex salts with triphenylmethylphosphonium and with triphenylmethylarsonium showed growth accessories and etch pits but also slide steps which could be provocated intentionally as by local deformations with a needle point as by axial pressure deformation. The orientation of the slide steps in different crystal faces leads to (010) as the slide plain. Axial pressure normal to the (010) faces caused unexpected strong shear deformation, which could serve for the estimation of the slide directions parallel to [101] or [101]. The sliding and cleaving parallel to (010) is favoured by the crystal structure, for (010) being parallel to a „smooth”︁ net-plain not crossed by overlapping molecules.     

化学气相沉积法制备石墨烯晶畴的氧气刻蚀现象研究

利用化学气相沉积法在抛光铜衬底上制备出六角形石墨烯晶畴,并对石墨烯晶畴进行氧气刻蚀.刻蚀完成后,利用光学显微镜和扫描电子显微镜观察到石墨烯晶畴表面的褶皱被刻蚀成网络状和短线状形貌的刻蚀条纹,并且刻蚀条纹的密度分布差异较大.通过电子背散射衍射测试证明了铜衬底的晶向与褶皱的形貌和密度分布有密切关系,不同的铜衬底晶向会影响褶皱的形貌和密度分布.通过改变刻蚀时间和刻蚀温度,发现刻蚀温度对石墨烯的氧气刻蚀具有更重要的影响,当刻蚀温度高于250 ℃时,刻蚀速率明显提高.这种氧气刻蚀方法,为观察石墨烯表面褶皱的形态和密度分布提供了一种便捷的途径.     

Kerr Effect Studies in Liquid Crystalline Substances

The static and dynamic pretransitional behaviour was investigated by the electric Kerr effect in three nematic substances above the clearing point. The measured Kerr constants exceed that of nitrobenzene by more than a factor of 50…︁ 80 which are due mainly to pretransition phenomena and the strong dipole moments of the compounds. A formula is given for the calculation of the Kerr constant. The latent heat at the nematic-isotropic phase transition calculated from the temperature dependence of the Kerr constant by means of the Landau theory is in good agreement with the experimental values. Size and relaxation behaviour of pseudonematic domains are determined by Kerr effect measurements.     

Polar structure of N,N′-dimethylurea crystals

In the crystal structure of N,N′-dimethylurea, there is one molecule in the asymmetric unit, but with two halves related by an axis of pseudosymmetry. All nonhydrogen atoms of each molecule lie on a plane. In the crystal the molecules are held together by hydrogen bonds in the direction of the “a” axis. They form parallel rows with the same sense orientation, so that the crystal has a polar structure.     

Die Orientierung von Silberaufdampfschichten auf Wolfram

The Epitaxy of single silver crystals on tungsten single crystal surfaces in UHV is studied. The substrate was a W-ribbon recrystalized at high temperature. After the high temperatur treatment many differently oriented single crystal regions appeared on substrate surface. The orientation of the single crystal regions crystal regions was determined by means of etch pits. The experimentally found change of the silver crystal orientation with changing the substrate orientation gives the following epitaxial relationship It is shown that the observed epitaxial growth of silver on tungsten is due to the structural similarity of the parallel growing Ag/W-planes.     

Electric Field Effects on Sensitized Fluorescence Emission From Nematic Liquid Crystal Cells

Remarkable electric field effects were observed with sensitized fluorescence intensities of a perylene derivative (BPDC), when rectangular AC waves (up to 10 V_0-p ) were applied to nematic liquid crystal cells containing a sensitizer (C307 or AHDA) and an acceptor (BPDC) as the guest molecules. Measurements of the fluorescence lifetime of AHDA indicated that the rates of energy transfer from AHDA to BPDC were independent of the electric fields. The electric field effects completely changed the sign when the intensity variation detected at vertical direction was compared with that at the horizontal direction with respect to the liquid crystal cell. The observed electric field effects on the sensitized fluorescence were consequently suggested to come from the orientation control of transition dipole moment of BPDC through the molecular alignment of the liquid crystal. The suggestion was confirmed by the measurement of dichroic absorption of the donor-acceptor systems.     

Packing motifs as predictors of the propensity of antibody fragments to crystallize

A recurring theme in the crystallization of antibody fragments in our laboratory has been a packing pattern involving formation of intermolecular, antiparallel β-pleated sheets across two-fold axes. The most common motif is the antiparallel stacking of constant (C) domains of light (L) chain dimers or Fab molecules. Here, cross-molecule six-stranded sheets are produced by hydrogen-bonding interactions of three-residue polypeptide segments (triads), in the i, i+2 and i+4 positions of the final strands (designated 3–3) of the three-chain layers from two adjacent molecules. In the variable (V) domains the triads are supplied by the first strands (4–1) of the four-chain layers and the resulting cross-molecule sheets contain eight strands. The latter type of packing is more likely to be seen in crystals of Fv fragments (V domains only) than in those of L chain dimers or Fabs. Amongst the triads from either the V or C domains, there are on average four sets of backbone carbonyl and amide groups within hydrogen bonding distance (<3.2 Å) of each other. In at least one example, the adjacent antiparallel strands are sterically aligned, but only two of the appropriate sets of atoms are sufficiently close to meet the distance criteria for intermolecular hydrogen bonding. These observations have been used to construct a list of rules for predicting which types of L chain dimers, Fab and Fvs are likely to crystallize in these packing patterns.     

Dielectric investigations of a binary system with strong correlation of the molecules

Dielectric measurements on oriented liquid crystalline substances in the frequency range from 10 Hz to 100 kHz have been carried out. The static dielectric constant parallel to the director of both single components shows a strong decrease in relation to the expected value. In the case of the swallow-tailed compound this effect can be described by assuming an equilibrium between statistically and antiparallel oriented molecules. The second component has an extremly strong tendency to be antiparallel oriented. In the mixture of both a continuous change of the correlation was observed.     

Crystal and molecular structure of an amber polymorph of 4-methyl-2-nitroacetanilide (MNA)

The crystal and molecular structure of a third polymorph of 4-methyl-2-nitroacetanilide has been determined by X-ray diffraction. It crystallizes in the monoclinic system, space groupP2₁/c,Z=4,a=10.158(2),b=11.635(2),c=8.041(2) Å and=94.55(2)°. Using 1027 unique reflections, the structure was solved by the method of vector verification and refined by full-matrix least squares, which gave convergence toR=0.080. The structure consists of nearly planar molecules, all approximately parallel to one another with their longitudinal axes parallel tob. The amide group forms an intramolecular hydrogen bond with the nitro group. Molecules related byc-glide are stacked alongc in a very distinct columnar form. The columns are held together by dipole-dipole interactions between close antiparallel carbonyl groups or between close antiparallel nitro groups.     

Alignment Properties of Some Polymeric Layers

Orientational properties of rubbed thin polymer layers with vinylic backbone and different side groups are presented. Some of these polymers lead to the alignment of LC molecules parallel to the rubbing direction whereas other polymers impose a perpendicular orientation. The stability of the obtained alignment as function of the temperature was tested.     

Bilayered Super-Structures of Antiferroelectric Mesogens

Abstract

Crystal structures of two antiferroelectric mesogens, TFMHPBC and MHPBC-10, were analysed by an X-ray diffraction method. In both crystals, mesogen molecules formed a herringbone structure which was essentially the same as that proposed for the antiferroelectric liquid crystal phase. Because of the crystallographic 2₁-symmetry along the b-axis, only the b-axis component of the dipole moment remains in a smectic layer. Since the dipole moment in the next layer has the same magnitude but the opposite direction, both crystals show no dipole moment as a whole. These structural features observed in their crystal states seem to be conserved in their antiferroelectric liquid crystal phases which are just above their crystal phases.     

Röntgentopographischer Nachweis ferroelektrischer Domänen in LiNbO3-Einkristallen

Ferroelectric domains in LiNbO₃ single crystals can be imaged by means of X-ray topography because of the difference of the intensities reflected. It is shown that it is possible to attribute the structure factors ∥F(hkl)∥ and ∥F(h̄m̄l̄)∥, respectively, to the antiparallel domains differing from each other in the opposite polarization along the c-axis. The ratio determining the contrast of visibility in the topogram was calculated for the 006 and 0012 reflexion and for different wavelengths taking into account the anomalous dispersion. For CrKα-radiation resulted We succeeded in verifying experimentally the intersity ratio and the inversion of the contrast in the transition from the 006 to the 0012 reflexion. The higher structure factor of the 006 reflexion could be attributed to the positive dipole end.     

A Polymorph Crystal Structure of Hexaphenyl Observed in Thin Films

Highly oriented thin films of hexaphenyl — which are used in organic opto‐electronic applications — are characterised in terms of their crystal structures. Two different crystal structures of hexaphenyl (C₃₆H₂₆) are observed when the films are prepared by physical vapour deposition at various substrate temperatures. If the substrate is kept at room temperature, hexaphenyl crystallises within a structure which is already known from single crystal investigations. However, when the thin films are grown at a substrate temperature of 160°C a new crystalline phase appears. This structure was characterised by X‐ray and transmission electron diffraction. Due to the strong preferred orientation of the crystallites within the thin films, the lattice constants as well as main features of the new crystal structure could be determined. The lattice is indexed as monoclinic with: a = 7.98Å, b = 5.54Å, c = 27.64Å and β = 99.8°. The new crystal structure has high similarity to the already known crystal structure: Both structures are built by layers of hexaphenyl molecules, within one layer the aromatic planes of the hexaphenyl molecules are packed in a herringbone pattern. The characteristic feature of the new structure is that the long axes of the hexaphenyl molecules are arranged absolutely perpendicular to the layers, whereas, within the already known structure the long axes show an tilt angle of 17° to the layer normal direction.     

The numerical simulation of heat and mass transfer during growth of a binary system by the travelling heater method

The influence of convection and heat and mass transfer on the shape and position of melt/solid interfaces and on radial composition segregation is analysed numerically for the travelling heater method growth of a binary alloy in a vertical transparent ampoule. Results are presented for crystal and melt with thermophysical properties similar to Cd_xHg_1−xTe with the assumption that the pseudobinary CdTe-HgTe phase diagram is true. The two-dimensional axisymmetric heat transfer equation, hydrodynamical equation and convective diffusion equation are included in the mathematical model. The rates of crystal growth and dissolution are supposed to be proportional to the compositional supercooling in the melt near the interfaces. It is shown for the conditions when convection is absent that the interfaces are asymmetrically positioned respectively to the heater centre line. Intensive convection makes their position more symmetrical but the length of the liquid zone greater. The flow pattern in the melt appears to be greatly influenced by solutal gravitational convection. The nonlinear dependence of the melt density on the temperature and composition are used in the model. The cases when speed of the heater is antiparallel (stable density stratification) or parallel (unstable stratification) to the vector of gravitational acceleration are considered.