Calculation of effective charges of ions in displacive-type ferroelectrics

The procedure of calculating the effective charges of ions in ABO₃-type ferroelectrics is considered, which is based on the comparison of two experimentally determined quantities: spontaneous polarization in a crystal and the parameters of the tensor of an electric-field gradient at the quadrupolar nuclei of A ions calculated based on the classical electrostatic model. The determined effective charges of ions in an LiNbO₃ crystal are considered.     

Annual Meeting of the American Chemical Society

Abstract

Ordered Fluids and Liquid Crystals was the focus of the two-day symposium on April 5 and 6 in Anaheim, California that was sponsored by the Division of Colloid and Surface Chemistry, during the Annual Meeting of the American Chemical Society. The organizers were 5. Rananavare of the Oregon Graduate Institute and R. Shashidhar of the Naval Research Laboratory who did a magnificent job in arranging a program that was faithful to the title and addressed issues on nearly all topics of liquid crystals research and technology. Both days were packed with presentations that covered lyotropics, ferroelectrics, liquid crystal polymers, molecular theory, new materials, and, of course, applications.     

Structural,ferroelectric and magnetic properties of Bi0.85Sm0.15FeO3 perovskite

Room temperature crystal structure, ferroelectric and magnetic properties of polycrystalline Bi_0.85Sm_0.15FeO₃ samples were investigated. X‐ray diffraction study shows that the compound possesses a dominant PbZrO₃‐like orthorhombic structure with √2a ×2√2a ×2a superlattice (a is the parameter of the cubic perovskite subcell). In contrast to piezoresponse force microscopy data demonstrating some features characteristic of ferroelectrics, polarization vs. electric field measurements reveal the behavior expected for nonpolar materials. Investigation of magnetic properties confirms that 15% samarium substitution suppresses the spin modulation typical of BiFeO₃ and induces the appearance of spontaneous magnetization. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Size effect in displacive ferroelectrics

It is ascertained that the size effect in thin ferroelectric films is related to the spatial correlation of ferroelectric polarization, depending on boundary conditions. The influence of the size effect on the permittivity of displacive ferroelectrics is described. It is shown that the size effect manifests itself differently for different orientations of the polarization vector with respect to the boundaries of a ferroelectric layer.     

Pseudosymmetry and ferroelectric phase transitions in the KTP structure type

The characteristic pseudosymmetry observed in the crystal structures of KTiOPO₄-type ferroelectrics have been analyzed. The low-symmetry phases of all the crystals under consideration are characterized by a pronounced pseudosymmetry. The transition temperature monotonically decreases with an increase of the pseudosymmetry parameter of the crystal structure of the ferroelectric phase. The approach suggested to evaluate the transition temperature is more precise and universal than the Abrahams-Jamieson-Kurtz criterion.     

On the modelling of a pretilted nematic cell

The authors have found that the perturbation method may be successfully applied in order to get approximated analytical solutions very close to the exact ones for the tilt angle distribution inside a pretilted nematic liquid crystal cell, if the elastic ratio k₁₁/k₃₃ is «, ～, or » 1. Hence, both the cell impedance and transmittance may be easily predicted for all commonly used nematics.     

Mathematical model of compensation for dispersion of light polarization states in electrooptical systems based on chiral liquid crystals

Spectral dispersion of the polarization states occurs after passage of linearly polarized light through a thin layer of a chiral nematic crystal along the helical axis. It is clearly pronounced for crystals with a high optical anisotropy and a helical pitch somewhat larger than the light wavelength. The results of numerical analysis of the spectral dependence of polarization states at the output of the liquid crystal layer and the method for compensating for the dispersion are presented for the first time. It is shown that polarization dispersion can be eliminated using phase plates of two types, one of which should have a high anomalous birefringence dispersion. The possibility of developing fast light modulators operating in a wide spectral range is discussed.     

Macroscopic Alignment Control of Liquid Crystal Molecule in Optical Film for 3D Display

The optical film made of liquid crystal molecules can have useful anisotropic optical properties by controlling the alignment of molecules. In order to apply those films to commercial display, we have to know the way to control the thickness, appearance, refractive index, adhesion, and many properties in macroscopic scale. In this presentation, we show how can we control alignment of liquid crystal molecules in macroscopic scale and make the Film Patterned Retarder (FPR^TM) used in the polarization type stereoscopic 3D-displays.     

Photo Controlled Surfaces in Rheology of Liquid Crystals

The modified method for shear viscosity measurements of nematic liquid crystals (NLC) oriented by surfaces is described. The main modification is concerned with usage of photoalignment technique for preparation of mono-domain samples with an orientation controlled by a polarization state of the secondary light irradiation. It provides the azimuthal rotation of a sample at any desirable angle. Thus different geometries of shear flows can be realized after filling the measuring cell with a liquid crystal. The method is experimentally approved at investigation of Poiseuille decay flows of 4-pentyl-4-cyanobiphenyl (5CB) through the channels with a rectangular cross section. The proposed construction of the cell with different surface treatment provides measurements of the three principal viscosities coefficients (Miesowicz’ viscosities) with additional usage of electric field. A small amount (less than 0.2 ml) of a liquid crystal needed for measurements and simple measuring procedure makes the method to be useful for rheological studies of newly synthesised liquid crystal materials.     

Physical Properties of Ferroelectric Liquid Crystals Incorporating a Boron Atom

Physical properties of a new type of ferroelectric liquid crystal incorporating a boron atom have been studied. Spontaneous polarization, tilt angle and chiral pitch of the new ferroelectric liquid crystals are compared with those of other types of ferroelectric liquid crystals. The alternation of the Ps direction and the twist sense is observed as the chiral center is moved further away from the core.     

Optical Rotatory Power of a Cholesteric Liquid Crystal for Obliquely Incident Light Using Multiple Scaling

The axis of polarization of light propagating through a cholesteric liquid crystal rotates with the angular rotation per distance traversed termed rotatory power. The rotation is due to the helical array of the molecules rather than due to the individual molecules as is found, for instance, with light being propagated through a sugar solution. The De Vries model for rotatory power of a cholesteric liquid crystal is generalized using multiple scaling theory to include light traveling at an oblique angle to the pitch axis. The resulting expression is tested experimentally using a mixture of cholesteryl oleyl carbonate and cholesteryl chloride.     

Molecular symmetry and ferroelectricity in pure organic molecular crystals

The molecular symmetry of all the existing organic ferroelectrics was analyzed with an intention to develop a criterion to search for new low molecular weight organic ferroelectric. We present here a summery of the analysis; it is conclude that in order to exhibit ferroelectricity in crystalline state the molecules should necessarily poses symmetry higher than C₁, since this appears to be an important requirement to ensure reorientability of the electrical polarization of the crystals. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

晶体中负离子配位多面体结晶方位、形变与晶体压电、铁电性

本文研究了压电、铁电晶体中负离子配位多面体的结晶方位与形变,提出了压电晶体中同一种负离子配位多面体的结晶方位是一致的.在铁电晶体中,负离子配位多面体发生形变,伴随着晶体发生顺电-铁电相变,并从这一基本过程出发,对铁电体相变的形成机理进行了讨论.     

Formation of unusual nonferroic domains in TeO2 single crystals under external electric field

The formation of unusual slowly relaxing domains under an external dc electric field has been revealed in paratellurite (TeO₂) crystals. These domains differ from those arising in ferroics (ferromagnets, ferroelectrics, ferroelastics, etc.). The effect is characterized by the existence of a threshold field strength (at which domains begin to be formed) and long equilibrium settling times (up to a few hours, depending on the electric field strength). A crystal returns to the initial single-domain state also after a few hours after the field is switched off. High-resolution triple-crystal X-ray diffractometry has revealed that domains retaining the paraelastic tetragonal phase rotate with respect to each other in space without changing their lattice parameter. The domain sizes are 2–4 mm, depending on the field strength. Currently, the exact mechanisms of domain formation are unclear. Possible reasons for the formation of these defects and an analogy of the observed effects with the behavior of liquid crystals under electric field are discussed.     

Polarization mechanism of LiNbO3 crystal due to thermoelectric power and current-induced electric fields during growth via micro-pulling-down method

The mechanism of polarization due to thermoelectric power and current-induced electric fields during the growth of LiNbO₃ crystals was studied using a micro-pulling-down method. With no applied electric current, a +c single-domain crystal was grown regardless of the domain orientation of the seed crystal. This +c domain growth was consistent with the direction of the electric field caused by the thermoelectric power in the liquid, despite an opposing electric field in the solid due to the opposite sign of the Seebeck coefficient. Thus, it was the electric field in the liquid that determined the domain structure of the growing crystal. On the other hand, when a current was applied from the melt to the crystal, a −c domain crystal was grown. The electric current required for this domain inversion to occur became larger as the temperature gradient in the solid phase decreased. This shows that the electric field in the solid phase became large enough to induce domain inversion from +c to −c through a combination of the thermoelectric power in the solid phase and current-induced electric field.     

Second-harmonic generation in vinylidene fluoride-trifluoroethylene copolymers doped with donor-acceptor molecules

The temperature dependence of the second-harmonic generation in a vinylidene fluoride-trifluoroethylene ferroelectric copolymer doped with 5 wt % of a noncentrosymmetric 4-anilino-4′-nitroazobenzene chromophore is analyzed. The second-order susceptibility χ⁽²⁾ in the ferroelectric phase of these copolymers appears to be twice as high as in the poly(methyl methacrylate) with the same chromophore content. The effect is attributed to the presence in ferroelectrics of a local electric field associated with the spontaneous polarization. The intensity of the second-harmonic generation in both ferroelectric and paraelectric phases correlates with the variation of the surface potential.     

OPTICAL TRANSMISSION AND POLARIZATION SWITCHING PROPERTIES OF A FERROELECTRIC LIQUID CRYSTAL CELL WITH A STABLE TWISTED STATE

Abstract

Ferroelectric liquid crystal cells with a stable twisted state were prepared. Dynamic processes of molecular reorientation in a stable twisted state under an applied electric field have been investigated by observing the optical transmittance and polarization switching current with simultaneous observation of the texture change.     

Birefringence of binary liquid crystalline mixtures of MBBA and PPMAECOBA in TCM,interferometric assessment

Abstract

The mutual influence of two mesogenic components, namely a nematic liquid crystal N-(p-methoxy-bezylidene)-butylaniline (MBBA) and a lyotropic liquid crystal poly-(phenyl-methacrylic) ester of cetyloxybenzoic acid (PPMAECOBA) in tetra-chloromethane (TCM) is studied in this paper. The ordinary and extraordinary refractive indices of the mixtures, nematic liquid crystal and lyotropic liquid crystal were measured at 589.3?nm with a Rayleigh interferometer and some electro-optical parameters were computed. The main refractive indices, the birefringence, the main normalized polarizabilities and their difference are dependent on the mixture volumetric concentrations, proving the existence of the collective orientational interactions between the two types of liquid crystals.     

Experimental Solution of the Local Field Problem in Uniaxial Liquid Crystals†

A method of experimental determination of the Lorentz-factor tensor in uniaxial liquid crystals is suggested. The specific features of the local field tensor of the light wave in a nematic liquid crystal with low optical anisotropy have been investigated experimentally. Anisotropy of the local field and Lorentz-factor tensors has been found to decrease with the decreasing optical and molecular-optical anisotropies. These results appear to contradict the existing polarization theories of liquid crystals. The local field problem in cholesteric liquid crystals has been considered. A new approach to the local field problem in uniaxial liquid crystals with arbitrary optical anisotropy is developed and the experimental results are explained.     

Flexoelectric Behavior of a Bimesogenic Liquid Crystal

The effective flexoelectric coefficient and flexoelectric polarization are investigated for a bimesogenic liquid crystal CBC11CB with a twist bend nematic phase (N_tb). The effective flexoelectric coefficients for CBC11CB in the ordinary nematic phase (N) are obtained to be much larger in comparison to conventional calamitics, and is almost twice higher for CBC11CB than that reported for another odd symmetric bimesogen. The flexoelectric polarization (P_f) in the N phase is found to be almost 2.5 times higher than in rod-like molecules. Additionally, P_f is obtained to be much larger in the N_tb phase than in the nematic phase.