A Binary System of a Swallow-Tailed and a Double Swallow-Tailed Compound

The geometrical anisotropy in the shape of swallow-tailed ST liquid crystalline molecules results in an extremely strong tendency of an antiparallel order in the short range. Dielectric measurements on a binary system of a ST and a double ST compound show that even at a mole fraction of 0.5 this effect can be observed. The strong deviations from the statistical distribution of the directions of the molecular long axis is caused by the asymmetric repulsive forces and the dipole-dipole interaction.     

Binary Systems of Swallow-Tailed and Laterally Branched Liquid Crystals and their Dielectric Behaviour

Swallow-tailed liquid crystals were mixted with four laterally branched substances in order to investigate the influence of molecular geometry on the correlations of molecules in the short range order. Besides the isobaric phase diagrams the results of dielectric measurements on oriented probes in the frequency range from 10 Hz to 100 kHz are discussed. Microscopic as well as dielectric investigations show that laterally branched molecules destroy the antiparallel arrangement of swallow-tailed substances and in this way destabilize the smetic A phase. It is possible to put in order laterally branched substances according to its “destroying” influence.     

Orientation of Thermotropic Liquid-Crystalline Polyesters in High Magnetic Fields

Strongly oriented poly-crystalline samples of polyesters containing mesogenic moieties and flexible spacers in the main chain have been produced by slowly cooling the molten samples in magnetic fields of up to 16 Tesla. This simple method appears to be successful for those polymers forming a “true” nematic phase over an extended range of temperatures. Cholesteric polymers are not oriented. Mesogenic polymers (such as SEB-5, SUC-5, and SEB-7) which pass only through a state of partial parallel ordering of chain segments before cystallizing cannot be oriented macroscopically. In these samples crystallization may interfere with the chain alignment in the magnetic field. In contrast to low molecular weight liquid crystals, the isotropic-liquid crystalline transition of the polyesters investigated has a strong thermal hysteresis.     

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.     

Crystal and molecular structures of three ketoximes: (E)-O-hexanoyl phenyl 2-pyridyl ketone oxime (C18H20N2O2), (E)-O-octanoyl phenyl 2-pyridyl ketone oxime (C20H24N2O2), (E)-O-myristoyl phenyl 2-pyridyl ketone oxime (C26H38N2O2)

The crystal structures of the three title compounds have been determined by the X-ray structure analysis. The molecule of (E)-O-hexanoyl phenyl 2-pyridyl ketone oxime1 has an anti form at the N-O-C-C ester linkage, while the molecules of (E)-O-octanoyl phenyl 2-pyridyl ketone oxime2 and (E)-O-myristoyl phenyl 2-pyridyl ketone oxime 3 each have a syn form. Compounds2 and3 are crystallized in isomorphic bilayer structures, in which alkyl chains are packed in an antiparallel arrangement.     

Field-induced Colour Change of Liquid Crystalline Dyes

Planar oriented nematic layers and also partially oriented smectic C phases of certain diphenyl-1,2,4,5-tetrazine compounds due to their positive dielectric anisotropy can be oriented by an electric field (Freedericksz-transition). The orientation is connected with a distinct change of absorption respectively colour. The mechanism of the effect is discussed. For both liquid crystalline phases the response times and decay times were measured.     

Magnetic moments of Fe and Y in the FeBY glass forming system

The new Fe_71.2B₂₄Y_4.8 glassy alloy has been prepared in amorphous ribbon form via melt spinning and splat cooling. The high degree of sample homogeneity and structural quality of the ribbon is revealed by the observation of narrow linewidths in the spectra of the field derivatives of the absorbed microwave power in ferromagnetic resonance experiments. Combination of vibrating sample magnetometry and element-specific X-ray magnetic circular dichroism experiments at the L_3,2-edges of Y revealed an induced magnetic moment of 0.065 μ_B/atom for Y antiparallel oriented to an Fe magnetic moment of 1.83 μ_B/atom.     

Isotopic and Optical Studies of the Decomposition of Crystalline Dibenzoyl Peroxide

Mass spectral analysis of volatile products from thermolysis or photolysis of a solid solution of dibenzoyl peroxide (BPO) and 4.4′-dideutero BPO shows that most phenyl benzoate is formed intramolecularly, while most biphenyl is formed intermolecularly, presumably by radical-molecule reactions. The strong visible dichroism in uv-irradiated BPO crystals appears to be due to selective scattering of light polarized along [010], not to the selective absorption which would have indicated an oriented product with a strong visible chromophore. Thermal development of the dichroism may be due to radical chain reactions. Studies on selective decarboxylation of BPO molecules using plane polarized light are discussed in terms of the birefringence of crystalline BPO.     

Oriented Thin Films of the Low-Band-Gap Polymer PTB7 by Friction Transfer Method

Low-band-gap polymers are promising materials for organic photovoltaic application. We prepared an oriented thin film of a low-band-gap polymer, PTB7, by friction transfer method. The oriented film showed a strong dichroic absorption in the whole visible range. The orientation of molecular chain was characterized by two-dimensional grazing incident X-ray diffraction (2D-GIXD) with synchrotron radiation. 2D-GIXD results showed PTB7 molecular plane parallel to the substrate plane, i.e., “face on orientation.”     

Stabilization and destabilization of the SA phase in mixtures of 4-nitrobenzyl-2,5-bis(4-n-alkyloxybenzoyloxy)benzoates

4-Nitrobenzyl-2,5-bis(4-n-alkyloxybenzoyloxy)benzoates exhibit S_A phases which are characterized by an antiparallel packing of the molecules and a partial intercalation of the terminal alkyl chains. In binary systems of selected rod-like components the S_A mixed phase can be stabilized or destabilized depending on the molecular length of the rod-like molecules. This unusual behaviour is mainly due to steric interaction and can be interpreted by a simple packing model.     

The Influence of the Shape and the Incompatibility of Molecular Moieties on the Structure of Smectic Mesophases

For selected compounds with perfluorinated chains the structure of smectic phases was investigated by X-ray diffraction measurements. For rod-like compounds with only one terminal perfluorinated chain the incompatibility of the hydrocarbon and the fluorocarbon moieties leads to a segregation of both parts which can be considered as microphase separation. For swallow-tailed compounds the structure of smectic phases depends on the position and the length of the perfluorinated chains. If the steric interaction dominates an antiparallel packing of the molecules within the smectic layers is favoured. If the incompatibility of the hydrocarbon and fluorocarbon moieties is of importance the smectic layers are distorted and at lower temperatures the layer structure is transformed into a columnar structure.     

Fast ultrasonic wavelength tuning in X-ray experiment

A method of tuning (scanning) X-ray beam wavelength based on modulation of the lattice parameter of X-ray optical crystal by an ultrasonic standing wave excited in it has been proposed and experimentally implemented. The double-crystal antiparallel scheme of X-ray diffraction, in which an ultrasonic wave is excited in the second crystal, is used in the experiment. The profile of characteristic line k_α1 of an X-ray tube with a molybdenum anode is recorded using both the proposed tuning scheme and conventional mechanical rotation of crystal. The results obtained by both techniques are in good agreement.     

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.     

Phase transition and domain structure in iodoform crystals

Phase transition in an iodoform crystal has been established from the temperature dependence of the Raman spectra of “external” crystal vibrations at 260 K. The transformation mechanism is interpreted under the assumption on the existence in the crystal of domains where dipole moments of the molecules have parallel or antiparallel orientations. Below the phase-transition temperature, the structure is more ordered, and the crystal consists mainly of domains with the parallel orientation of dipole moments of the molecules, whereas above this point, the domains with the antiparallel orientation of dipole moments prevail. The crystal as a whole is not a pyroelectric.     

Revelation of microtwinning in hydrothermally grown synthetic quartz by growth structures

Growth structuress on prism surfaces of synthetic quartz grown by hydrothermal crystallization technique, indicative of microtwinning in them are described, illustrated and discussed. The microtwins are revealed by the misorientation of the growth structures, and it is observed that microtwinning is manifested in different forms. Twinned domains appear in the form of strictly oriented, square striated (stepped) structures with inclined surfaces, seemingly overlapping square striated structures, square and irregular regions demarcated by black line-like boundaries, patch-like structures enveloping irregularily bound domains, tongue-like terraces bordered by irregular line of discontinuity of varying height and the regions with both differently shaped as differently oriented growth structures. From the orientations of the square shaped growth structures, it is shown that crystals consist of domains which are in twin relationship to the main crystal. Twins by rotation around an axis perpendicular to (10ī0) with rotation angles of 60°, 90°, and 180° have been observed. Critical examination of some twin boundaries delineated by black line-like matter, with the application of multiple beam interferometry and FECO reveal that the black boundary is a ridge (line-elevation). The line elevation along such a twin boundary is explained to be as a result of preferential adsorption of impurity atoms along the trace of the boundary. It is further observed that discontinuity in crystalline structure along the twin boundary leads to growth forms with distorted morphology thus establishing strong dependence of growth forms on the underlying crystalline structure. Causes for generation of the microtwins (twinning on a microscale) in synthetic quartz are discussed.     

添加Zn~(2+)对六角晶锶铁氧体结构与磁性能的影响

通过传统的陶瓷化工艺来制备La、Co、Zn复合取代的锶铁氧体,重点研究Zn~(2+)对锶铁氧体结构及磁性能的影响.采用X射线衍射对锶铁氧体结构进行了分析.结果表明:随着Zn~(2+)的增加,锶铁氧体主体仍为六角晶结构,晶格常数a逐渐增大,而晶格常数c先增大后减小.扫描电镜(SEM)分析显示,铁氧体晶粒主要呈片状,大多为规则的六边形.随着Zn~(2+)的增加,晶粒的尺寸逐渐增大.磁性能测量显示:当取代量x=0.15时,样品的Br和Hcj分别为436 mT、343 kA/m,这说明添加适量的Zn~(2+)会有效的降低反平行离子磁矩,提高饱和磁化强度, 进而改善锶铁氧体的磁性能.     

Attractive Interactions of Spin Polarons in the Quasi-One-Dimensional Spin-Density-Wave Background

Abstract

The dynamics of spin polarons (spin bags) in the quasi-one- dimensional spin-density-wave background is numerically studied using the extended Peierls-Hubbard model. Interchain couplings are incorporated to the model as a staggered magnetic field. Time-dependent unrestricted Hartree-Fock equations and Newtonian equations are solved using Suzuki's decomposition of exponential operators. In the Hubbard model, the spin polarons with antiparallel spin polarizations attract each other. When the nearest-neighbor Coulomb repulsion is included, the interaction changes to a repulsive type. Further addition of an intersite electron-phonon coupling leads to an attractive interaction.     

Investigation of domain structures in PZNT93/7 crystals by chemical etching

The domain structures in 0.93Pb (Zn_1/3Nb_2/3)O₃‐0.07PbTiO₃ (PZNT93/7) crystals were investigated by chemical etching technique. Original antiparallel 180° domains of size 20–40 μm were observed on the surface of as‐grown PZNT93/7 crystal. It was found the domain states are sensitive to the stress field induced by mechanical processing or impurities. As the composition of PZNT93/7 crystal was located near the morphotropic phase boundary, various domain configurations were observed. On the <001>‐oriented wafer, the etched a ‐ and c ‐domains revealed homogenous and island‐like patterns. The c ‐domains consist of upper and lower height regions corresponding to the tail and head of the polarization, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Properties of Some Polarized Chromophores in Polyvinylpyrrolidone

Poor miscibility of highly polarized chromophores to polymer matrices is a crucial issue for EO polymer development. By using poly(N-vinylpyrrolidone) (PVP) as a matrix polymer, we were able to prepare the uniform film containing ammonium-borate type zwitterions or Disperse Red 1 (DR1). For zwitterions, corona poling was unsuccessful probably because of hygroscopic property of PVP and/or strong intermolecular interaction to form the antiparallel dimers. For DR1, we found apparent color change originated from the H-aggregate formation assisted by moisture, which is usable to detect humidity. On the other hand, a passivation layer is necessary to use PVP for the EO applications.     

Dislocation Multiplication in the Flexoelectric Distortion Grid with an Increase in Electric Field Amplitude

It is shown that a plane one-dimensional flexoelectric grid well simulates the processes of the nucleation of defects (dislocations) and an increase in their number in crystals under increasing mechanical stresses in the crystal lattice. The latter rise with an increase in electric field above the instability threshold and the corresponding growth of the modulations of the flexoelectric-grid spatial structure. Parameters of these modulations are obtained for various boundary conditions at the surface of a nematic film oriented in one direction. The previous data on the dislocation patterns in strong fields are explained. The applicability of the dislocation theory to the planar one-dimensional grid is shown.