The Use of X-Ray Diffraction in the Study of Thermotropic Liquid Crystals With Rod-Like Molecules

The paper first reviews briefly the various kinds of information that can be obtained from x-ray diffraction studies. Next, specific examples are given from the literature of determinations of each of the different kinds of data.     

X-Ray Diffraction Studies of Liquid Crystalline Polymers

Thermotropic liquid crystalline copolyesters display an ordered fluid phase at elevated temperatures which permits the development of unusually high orientation at ambient temperatures. The transition which occurs upon cooling the high temperature nematic liquid crystalline phase very rapidly (e.g. fiber spinning) results in a polymeric glass with nematic structural order. Annealing increases structural order from the nematic glass toward ideal three dimensional crystalline order. Precision X-ray diffractometry has been used to directly observe changes in structural correlations which occur with annealing. Structural transitions range in character in these copolyesters from no change upon annealing to a transition from fully two dimensional to fully three dimensional structural order. Increased three dimensional order also results in substantially increased first order character of the high temperature phase transition. The chemical structure of co-monomers determines, in part, the final degree of dimensionality.     

Simultaneous X-Ray Diffraction and Differential Scanning Calorimetry (XDDSC) in Studies of Molecular and Liquid Crystals

The technique of simultaneous X-ray diffraction and differential scanning calorimetry (XDDSC) is introduced into the field of molecular and liquid crystals. The technique allows fast time-resolved X-ray diffractograms to be recorded simultaneously with the recording of heat flow into or out of the specimen during a heating, cooling or isothermal scan. Structural transformations, as revealed by changes in the diffraction pattern, can thereby be unambiguously correlated with thermal events such as endo-or exo-therms or changes in heat capacity. Small and wide angle diffraction can be monitored. Sample pans and cuvettes made of boron nitride or graphite are used, both materials combining high transparency for X-rays with good thermal conductivity. Selected examples of application of the technique are presented.     

X-Ray Topography of Perfect Crystals using the Laue-Laue Three-Beam Case of Diffraction

A new kind of experiments is possible using very good collimated X-rays with a divergence of the incident beam smaller than the diffraction curve (rocking curve) of perfect crystals. Some examples are shown for the three-beam case of diffraction where both reflections are in Lauegeometry. The modification of the structure factor of a two-beam case reflection in the neighbourhood of a three-beam case causes a modification in the intensity distribution inside the Borrmann fan, which can be seen in section topographs. The energy flow in the three-beam case can be made visible in experiments with a very narrow incident beam with a 30 × 30 μm² cross-section.     

Diffraction Investigation of Transient Processes of Domain Formation in Liquid Crystals

A coherent light diffraction method for investigation of domain formation in liquid crystal is proposed. The good agreement between theoretical predictions and the experimental results has been demonstrated.     

Optimization of the Growth Technology for Crystals with Garnet Structure Based on X-Ray Diffraction Data

High efficiency of the methods of double-crystal X-ray diffractometry (DCXRD) and topography for improving the growth technology of highly homogeneous crystals has been demonstrated on the example of gadolinium gallium garnet (GGG) single crystals. The main types of structural defects observed in Czochralski-grown GGG crystals are found to be macroscopic inhomogeneity of composition distribution, caused by the facet effect manifestation; microinhomogeneous distribution of impurity and main components of the composition in striations; dislocations; and second-phase inclusions. The relationship between the type and density of newly formed defects and the technological conditions for crystal growth are considered. Optimization of the composition of crystals and their growth technology made it possible to obtain high-quality dislocation-free crystals of GGG and complex-substituted garnets on its basis for magneto-optical and microwave devices, elements of solid-state lasers, and other applications.     

Investigation of the Cholesteryl Oleate Mesophases by X-Ray Diffraction

The following phase transitions occurring in the cholesteryl oleate were investigated by X-ray diffraction: solid-isotropic liquid, isotropic liquid-cholesteric, cholesteric-smectic.

The sample purity was 98%. Strong pretransitional effects were observed at the solid-isotropic liquid phase transition and at the cholesteric-smectic phase transition.

At this last transition the longitudinal coherence length ξ_∥ appears to diverge as the critical temperature is approached in the cholesteric phase, whereas the transversal coherence length ξ_τ increases at the critical temperature in the smectic phase in an abrupt way. It seems from the temperature dependence analysis of the angular position of the low angle peak that the smectic phase is a smectic A phase and that a progressive melting of chains occurs at the temperature increases.     

Methods of Coherent X-Ray Diffraction Imaging

Crystallography Reports - Methods of coherent X-ray diffraction imaging of the spatial structure of noncrystalline objects and nanocrystals (nanostructures) are considered. Particular attention is...     

Dynamic Layer Response under Electric Field in Antiferroelectric Liquid Crystal Cells Measured by Synchrotron Microbeam Time Resolved X-Ray Diffraction

Abstract

Direct observation of the local layer response of an antiferroelectric liquid crystal to the step form electric field has been carried out by a time resolved synchrotron X-ray micro diffraction measurement. When an electric field was changed from high voltage to OV, corresponding to the ferroelectric to antiferroelectric phase transition, the local layer transformed from the bookshelf to the quasi-bookshelf structure within 0.3 ms. The horizontal chevron structure was found in both the phases, though the decrease in the horizontal chevron angle was observed during a period of 0.2 ms after turning off the electric field. In the antiferroelectric to ferroelectric phase transition process (from OV to high voltage), the layer structure transformed to the bookshelf within 0.04 ms.     

Neutron Diffraction Investigation of Langanite Crystals

The upper and lower parts of a single crystal with a nominal composition La₃Ga_5.5Nb_0.5O₁₄grown by the Czochralski method (Ir crucible, 99%Ar + 1%O₂, growth direction) have been studied by neutron diffraction for the first time. The compositions of the upper and lower parts are found to differ ((La_2.935(2)□_0.065)(Ga_0.450Nb_0.550(3))Ga₃(Ga_1.965(4) □_0.035)(O_13.90(1) □_0.10) and (La_2.940(1) □_0.060)(Ga_0.590Nb_0.410(2))Ga₅(O_13.82(1) □_0.18), respectively), and microtwin formation was observed in the upper part, with two unit cells related by the translation 1/2Z. Based on a comparison of the refined crystal composition and optical transmission spectra, the absorption bands at 30 000, 24 500, and 20 500 cm_-1 were assigned to divacancies (2 V _La^', 3 V _O^..), oxygen vacancies, and color centers, respectively. A relationship between the crystal color and oxygen content is established.     

Liquid Crystals Tomorrow

Abstract

This issue of Liquid Crystals Today is slimmer than usual, and so provides an excuse for an editorial. Copy for Liquid Crystals Today comes through a variety of channels: some items are sent unsolicited to the editor or members of the editorial board, while feature articles are often written as a result of one of the editorial team reading or hearing about some new development in liquid crystal science. The flow of contributions is, therefore, random, and one of the tasks of the editor is to try to ensure that, from this random flow, a steady stream of high quality material emerges to coincide with the publication dates. This time it hasn't happened, so rather than wait for articles in the pipe-line, we have decided to publish a thin edition of Liquid Crystals Today. We do have some excellent articles being prepared, and we expect to publish an extended edition next time to include these.     

Carbohydrate Liquid Crystals

Certain alkyl l-O and l-S glycosides with hydrocarbon chain lengths greater than hexyl have been shown to form thermotropic liquid crystals at temperatures between 60 and 100[ddot]C. These are believed to be members of a large class of alkyl and acyl carbohydrate mesogens which, apart from their intrinsic value as potentially useful solid-state materials, could provide a variety of structurally accessible model systems for studying the phase transitions in the cell-membrane glycolipids.     

Ferroelectric Liquid Crystals

A review of the ferroelectricity in liquid crystals is given, beginning with a discussion of the symmetry properties allowing a macroscopic polarization in some of the more ordered liquid crystal phases. The fundamental mechanisms behind the dipolar ordering are discussed in some detail. The two linear electric effects, ferro- and flexo-electricity, give rise to completely different phenomena. In the smectic C phase there is one ferroelectric coefficient and nine flexoelectric ones, giving independent contributions to the polarization of the medium. We further discuss helical and non-helical structures, doped and intrinsic infinite-pitch ferroelectrics and, finally, the rich potential of liquid crystal ferroelectrics aligned in the »book-shelfgeometry» for fast electro-optic devices.     

Polymer Liquid Crystals

Abstract

The field of polymer liquid crystals is encompassing an increasingly wide range of research areas and applications, as was demonstrated at a recent one day meeting devoted to the subject. The meeting was held in London at the Scientific Societies Lecture Theatre, and was organized by the Institute of Physics Polymer Physics Group and co-sponsored by the British Liquid Crystal Society and the Royal Society of Chemistry.     

High-Pressure X-ray Diffraction Studies on Organic Crystals

The high-pressure X-ray diffraction technique, commonly applied for inorganic crystals, is discussed in a view of its application for laboratory studies on molecular crystals of organic compounds – particularly when the weak X-ray scattering of such specimens causes experimental difficulties. To minimize the background-to-signal ratio, relatively big crystal samples (ca. 0.005 A³) were used and the mechanical properties of the gasket material were applied to obtain the optimum experimental conditions for pressure generation and lowering the background of reflections. The mechanical properties of tungsten, Inconel, and hardened steel gaskets are different and these properties can be helpful in planning high-pressure measurements with large high-pressure chamber to contain a big crystal sample. Several characteristic features of organic crystals – like their strong compressibility (depending on the nature of intermolecular interactions) and pressure-dependence of thermal vibrations of atoms – can have a direct impact on the conditions of high-pressure experiments. The properties of molecular crystals of organic compounds have been discussed on the basis of several high-pressure structural studies, mainly on simple cyclic β-diketoalkanes (enol forms). It has been attempted to assess the feasibility of investigating the details of molecular structure at high pressures by X-ray laboratory studies. A systematic pressure-induced change in the length of double C  O bonds is observed in this group of compounds, which can be correlated with the compression of the H-bonds involving the carbonyl groups.     

X-Ray Topographic Investigation of Dendritic Silicon Crystals

Using Lang and double-crystal X-ray topographic methods the dislocation structure of dendritic silicon crystals have been investigated. It is shown that the surface layers of these crystals have a more perfect structure than their bulk volume. The twin lamella is a dislocation-free formation and there are dislocation-free zones ∼ 1,5 mm in width in the volume of crystals.