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.     

Synthesis and powder X-ray diffraction of new Schiff-base liquid crystal

Liquid crystals (LCs) are soft materials that combine the fluidity of disordered liquids and the long-range orientational or positional order of crystalline solids, along one or two directions of space. A search for better stationary phases, including highly selective ones, is an important trend in chromatography development. Among the stationary phases under investigation are LC stationary phases that have been recently generated much interest. A variety of isomeric molecules were separated with high accuracy. Successfully, X-ray scattering is widely used to investigate and characterise the microscopic structure of most LCs. In this paper, a new liquid crystalline material that can be used potentially as stationary phase in gas chromatography was synthesised and characterised by means of nuclear magnetic resonance (NMR), infrared (IR) spectra, elemental analysis and X-ray diffraction.     

Sequence of Phase Transitions in Solid Mbba

Combined neutron diffraction and Raman scattering measurements were carried out on a series of different phases of the same MBBA sample. The change in the diffraction patterns and in the optical spectra are used to indicate the transitions between the different phases investigated in the whole temperature range of interest. Besides the isotropic and nematic liquid crystalline phases, 7 different solid structures were found and a schematic phase diagram was constructed to show the thermal procedur for producing these phases.     

Quasicrystalline and related phases in titanium based alloy systems

Amongst the non-aluminium based quasicrystalline alloys, investigations of titanium containing alloys in regard to the occurrence and stability of quasicrystalline phases have aroused considerable interest in recent years. Employing X-ray and TEM techniques a systematic investigation of the influence of substitution for Fe by Si and Ni on the stability of icosahedral phase in rapidly quenched Fe-Ti alloy has been carried out. The occurrence of metastable phases including commensurately modulated phase and the decagonal phase in Fe-Ti-Si system have been found. In addition, the occurrence ofstructural disorder manifested by arcs of diffuse scattering in diffraction patterns and anisotropy in the shape of idffraction spots has also been pointed out. It has been observed that 6 at.% of Si in Fe-Ti-Si system results in the formation of single-phase icosahedral quasicrystals. We have shown that contrary to some earlier reported results on Ti₂Ni, Ti₂Ni(Si) and Ti₅₆Ni₂₃Fe₅Si₁₆ alloys do not possess the icosahedral phase. In Ti₆₈Fe_26−xNi_xSi₆ alloy system, icosahedral phase formation ability is limited to the value of x < 9. The occurrence of icosahedral phase in these alloy systems has been analysed in terms of e/a ratio.     

Dynamical structure of fluid mercury: Molecular-dynamics simulations

We have carried out molecular-dynamics simulations for nonmetallic fluid mercury in liquid and vapor phases using a Lennard–Jones type effective potential and shown that the structure factors S(Q) and the dynamic structure factors S(Q, ω) of nonmetallic fluid mercury obtained by our MD simulations are in good agreement with recent X-ray diffraction and inelastic X-ray scattering experiments. We conclude from these results that, though the fluid mercury which shows a metal–nonmetal transition is a ‘complex’ fluid, the nonmetallic fluid mercury is a relatively ‘simple’ liquid, which can be well described by the single density-independent Lennard–Jones type potential.     

Differential anomalous X-ray scattering studies of amorphous Cd59As41 and Cd26As74

The local structure of amorphous cadmium arsenide semiconducting films has been studied by differential anomalous X-ray scattering. Intensity measurements were carried out on two samples, containing 41 and 74 at.% As, in the vicinity of the absorption K edges of both constituents using synchrotron radiation. The computational procedure, similar to that proposed by Warren for an amorphous sample with more than one kind of atom, was applied to obtain the structural parameters from the experimental data. It has been found that atoms in the amorphous CdAs films remain almost tetrahedrally coordinated and that the investigated films are chemically ordered. The structural changes going from cadmium- to arsenic-rich composition have been revealed. The differential anomalous X-ray scattering technique proved to be effective, providing the evidence for the CdCd and AsAs near neighbour correlations in Cd₅₉As₄₁ and Cd₂₆As₇₄, respectively. The simulations of the differential radial distribution functions have shown that for the amorphous film containing 41 at.% As the distorted tetrahedral structure, intermediate between the CdAs and Si III type structures, is adequate to account for the experimental data. At 74 at.% As, the atomic arrangement can be described satisfactorily by the structural model based on the tetragonal CdAs₂ structure. The structural parameters obtained from the present study and those previously derived using the extended X-ray absorption fine structure and conventional large angle X-ray scattering techniques are compared.     

The influence of the boundary conditions on the character of the phase transition nematic-smectic C

The possibility is investigated for inducing the intermediate smectic A structure in substances with nematic and smectic C phases by the boundary forces. Different geometries are used: homeotropic, homogeneous and tilted under a different angle toward the supporting substrates of the liquid crystal cell. At homeotropic and at tilted orientation where the molecules tilt toward the supporting substrates is different from the molecule tilt in the smectic layer, an intermediate smectic A structure is observed at the transition nematicsmectic C in a narrow temperature interval. In this case the phase transition nematicsmectic C extends and transforms into phase transition of second order. At homogeneous orientation and at tilted orientation of an angle equal to the molecule tilt in the smectic layer, the transition nematic-smetic C carried out in two stages, as partially it begins on the surface still in the nematic phase and terminates at the temperature of the phase transition N-Sc. The influence of the boundary forces depends essentially on the energy of the phase transition N-S_C of the substances.     

Liquid–liquid transitions,crystallization and long range fluctuations in supercooled yttrium oxide–aluminium oxide melts

Employing small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) combined with laser-heated aerodynamic levitation has enabled different transitions in supercooled yttrium oxide–aluminium oxide to be distinguished. These include liquid–liquid phase transitions as well as high temperature crystallization for different compositions. Prerequisites for avoiding crystallization in order to reveal polyamorphic phase separation in the supercooled state are established. We also show how the rise in SAXS intensity at low wavevectors can be used to identify correlation distances in long range fluctuations in high temperature melts. These distances appear to scale with melt viscosity and to extend temporarily during liquid–liquid transitions.     

Wide and small angle X-ray scattering measurements for expanded fluid Se accompanying the semiconductor–metal and metal–nonmetal transition

Wide and small angle X-ray scattering measurements for supercritical fluid Se in the wide density region from liquid to dense vapor have been carried out using synchrotron radiation at SPring-8. The large background for the small angle spectra with the previous high-pressure vessel was much suppressed using a new vessel with a large diamond window. We carefully measured wide angle X-ray scattering near the critical density and obtained the structure factor, S(k), and the pair distribution functions, g(r). By small angle X-ray scattering measurements, we succeeded in observing a medium-range fluctuation in the semiconductor–metal transition in expanded fluid Se.     

Structural aspects of solid-state amorphization in GaSb single crystals

As is well known, the GaSb I phase that is stable at room temperature is transformed under high pressures and temperatures into the GaSb II phase, which, being frozen in liquid nitrogen, becomes a metastable phase. The X-ray studies of GaSb II under heating showed the formation of two “thermally reversible” phases providing halolike X-ray diffraction patterns. It was also shown that the low-angle scattering spectrum also reversibly changes with the temperature and that macrocrystalline grains appearing during heating at elevated temperatures dissipate in the process of the subsequent specimen cooling. The structural states observed are interpreted as paracrystalline.     

Structural study of levitated liquid Y2O3 using neutron scattering

The local structure of liquid Y₂O₃ at 2800 K has been investigated by combining aerodynamic levitation and laser heating with the neutron scattering technique. Despite relatively low counting statistics, we have been able to determine the short-range order in the liquid. In particular, we found Y–O and Y–Y coordination numbers around 6 and 12, confirming our previous study by anomalous X-ray scattering. The unusually sharp main diffraction peak observed with X-ray measurements, and characteristic of a high degree of chemical ordering, is strongly reduced in the neutron structure factor and thus indicates that it is mostly due to Y–Y correlations.     

Light Scattering and Contrast in Thermally Addressed Liquid Crystal Displays

The effects of structural order on the light scattering characteristics of liquid crystal display cells containing smetic p-n-octyl-p′-cyanobiphenyl have been investigated. The scattering characteristics of different structures in the cell due to surface treatments are qualitatively analyzed for their effects on the contrast ratio when used in the thermally addressed liquid crystal display (TALC). Of the four structures investigated, i.e., homeotropic, fan-like, uniaxial rods and spherulites, the spherulitic structure has been found to exhibit the scattering profile best suited for high contrast because of the near-zero scattering near the incident beam. Based on the light scattering theory for spherulites, optimization of structural parameters such as size and order of spherulites, and device design such as collecting angle of projection lens for maximum display contrast has been predicted. Methods to improve contrast have also been discussed.     

Solid phases in cycloheptanol: Dynamic disorder and glass transitions

The stable solid polymorphism of cycloheptanol (C₇H₁₄O, for short C₇–OH) is revised in the present paper. Four solid stable phases, denoted on cooling from the liquid as phases I, II, III and IV are identified from X-ray diffraction measurements. Lattice symmetry of phase III has been unambiguously stated as orthorhombic. Diffraction patterns corresponding to phase IV have been also indexed as orthorhombic, but only in a tentative way. A detailed broadband dielectric spectroscopy study devoted to these phases has been made. Such an experimental technique has no revealed differences between phases III and IV and from specific-heat measurements, the III–IV phase transition has not been observed yet. Dielectric results seem to evidence that the dynamic disorder of phase III and/or IV should be intra-molecular attributed to changes between the possible distinguishable molecular conformations along with possible hydroxyl group rotations. The possibility of C₇–OH as conformational glass-forming material is discussed.     

Study of the defect structure of paratellurite crystal using multiwave diffraction and normal X-ray diffractometry methods

Multiwave X-ray diffraction has been used to study the structure of crystalline samples. A cycle of local measurements of rocking curves (RCs) of the (220) and (371) reflections under conditions of multiwave diffraction (MD) has been carried out in a paratellurite crystal. The data obtained are used to compare the sensitivity to structure defects of two-beam diffraction with that of MD, which makes it possible to study the X-ray wave phase. The comparison has revealed a higher ability of the phase-sensitive method to detect defects.     

Nano Molecular Self Assembly in the Formation of Induced Mesomorphism in Mixture of ChCl,DTAC, and EG

The multi-component system of cholesteryl chloride (ChCl), dodecyl trimethylammonium chloride (DTAC), and ethylene glycol (EG) exhibits very interesting liquid crystalline mesophases like cholesteric and SmA, SmC, and SmB phases sequentially when the specimen is cooled from its isotropic phase. These phases have been characterized by employing optical and X-ray studies. Pitch of the cholesteric phase has been calculated and discussed. Variation of pitch from the cholesteric phase to smectic phase is smooth and continuous. The temperature variation of optical anisotropy and electrical-conductivity has also been discussed. It has been found that wherever there is a phase transition, the value of electrical susceptibility changes appreciably.     

Brillouin light scattering study of polymeric glassy sulfur

We present here a study of the acoustic dynamics of polymeric glassy sulfur. The data have been collected in the GHz frequency range by means of Brillouin light scattering for different scattering geometries. The choice of the experimental setup allows us to obtain information on the refractive index, n, that comes out to be close to that corresponding to the high temperature polymeric liquid phase. The longitudinal acoustic excitations have been investigated as a function of temperature from deep into the glassy state up to the glass transition temperature. The temperature dependence of the sound velocity is compared to the one measured in the liquid phase. We find that the sound velocity of the glass can be linearly extrapolated from that of the polymeric liquid measured in the THz frequency range with inelastic X-ray scattering.     

Phase transition of nematic to smectic C through an intermediate boundary induced smectic A structure

An intermediate phase has been observed at the transition nematic to smectic C where a smectic A structure induced by the boundaries and a nematic phase exist simultaneously. The temperature interval of existence of the intermediate phase is of the order of one tenth of degree. Several conditions are necessary for its appearance: the latent heat L of the phase transition nematic-smectic C of the liquid crystal should be sufficiently small (L < k_BT_c); the nematic phase should undergo a bend deformation by the boundaries (possibly a twist one as well) and the transition nematic-smectic C should be carried out as slower as possible.     

Ferroelectric Liquid Crystals - Structure and Design

Interest in the synthesis of optically active smectic liquid crystals has increased considerably since the advent of a fast switching, bistable, electrooptic device configuration based on their ferroelectric properties. A number of structurally separate ferroelectric liquid crystal phases have been defined which possess differing properties. These types of phase can be utilized in different forms of application. The structures of the various ferroelectric smectic phases and the types of material which exhibit these modifications are discussed in detail. The design and engineering of materials to suit certain device criteria is related to the properties of the smectic ferroelectric phases. A relationship between the absolute spacial configuration of the optically active materials which exhibit ferroelectric smectic phases, the twist direction of the phase and the direction of the spontaneous polarization is developed.     

Structure and dynamics of levitated liquid aluminates

We used the aerodynamic levitation technique combined with CO₂ laser heating to study the structures of liquid CaAl₂O₄ and MgAl₂O₄ above their melting point with X-ray and neutron diffraction. We determined the structure factors and corresponding pair correlation functions describing the short-range order in the liquids. The combination of the two scattering techniques makes it possible to derive information not accessible with a single measurement. We also obtained information on the dynamics of liquid MgAl₂O₄ with inelastic X-ray scattering. In particular, we determined the sound velocity and the longitudinal viscosity.