Resonance phenomena in a ferroelectric liquid crystals

In a surface stabilized ferroelectric liquid crystal cell, optical transmission oscillations have been revealed accompanying mechanical vibrations caused by fast field reversal. Special bookshelf textures, so-called "rainbow", were used in the experiments. Temperature dependences of the oscillation parameters have been studied. The temperature dependence of the oscillation frequency suggests that the some oscillation resonances correspond to modes of the liquid crystals.

Temperature dependences of oscillation parameters in ferroelectric liquid crystals

Oscillation processes have been revealed in the course of reversible polarization study in ferroelectric liquid crystals. The oscillation frequency of polarization vector has been found to be from 1 to 30 kHz. The oscillation parameters were studied as functions of temperature. Temperature dependences of the oscillation amplitude and damping decrement have been measured.

Effect of elastic stresses on free oscillations in ferroelectric liquid crystals

It is shown experimentally that an elastic mechanical stress in a crystal structure is a necessary factor for the appearance of free oscillations of the director of a ferroelectric liquid crystal. Such a mechanical stress arises as a result of internal textural perturbations in the presence of regions with a different orientation of the director or is produced by external pressure applied to one of the cell plates in the appropriate direction. © 1999 American Institute of Physics.

New chiral E- and Z-isomers of the 1R,4R-2-arylidene-p-menthane-3-ones in induced cholesteric and ferroelectric liquid crystals

A novel pair of the E- and Z-isomeric 1R,4R-2-(4-heptyloxyphenyl)-benzylidene-p-menthan-3-ones has been prepared and the influence of distinctions in their molecular geometry on macroscopic properties of liquid crystal systems with the induced supra-molecular helical structure has been studied. The significantly lower helical twisting power of the chiral Z-isomer in comparison with that of E- one has been confirmed in the case of induced cholesteric systems based on 4-pentyl-4-cyanobiphenyl. The phase behavior and ferroelectric characteristics have been investigated for smectic-C* compositions based on the eutectic mixture of the homological 4-hexyloxyphenyl-4'-hexyloxy- and 4-hexyloxyphenyl-4'-octyloxybenzoates containing the novel isomeric chiral dopants. The spontaneous polarisation of the opposite signs induced by the isomeric chiral components has been revealed for the compositions studied. Distinctions in phase states, absolute values of the spontaneous polarization, smectic tilt angle and rotation viscosity of the systems obtained are discussed.

Photomechanically Induced Phase Transitions in Ferroelectric Liquid Crystals

We report optically induced phase transtions occurring in two different host ferroelectric liquid crystals; SCE13 a multicomponentmixture optimised for room temperature performance, and CE8 a single component liquid crystal. These act as host liquid crystals for a novel guest azo dye, which can be made to photoisomerise using low power density U.V. illumination, resulting in dramatic changes in sample properties. We have shown that the magnitude of spontaneous polarisation of systems can be isothermally and reversibly induced or reduced, with the consequent appearance or disappearance of optical switching hysteresis. We discuss the parameters controlling the behaviour of the systems under U.V. illumination and suggest mechansims by which the transitions may occur. © 1993, Taylor & Francis Group, LLC. All rights reserved.

Interplay between proton ordering and ferroelectric polarization in H-bonded KDP-type crystals

The origin of ferroelectricity in KH2PO4 (KDP) is studied by first-principles electronic structure calculations. In the low-temperature phase, the collective off-centre ordering of the protons is accompanied by an electronic charge delocalization from the near and localization at the far oxygen within the O-H...O bonds. Electrostatic forces. then, push the K+ ions towards off-centre positions, and induce a macroscopic polarization. The analysis of the correlation between different geometrical and electronic quantities, in connection with experimental data. supports the idea that the role of tunnelling in isotopic effects is irrelevant. Instead, geometrical quantum effects appear to play a central role. (C) 2001 Elsevier Science B.V. All rights reserved.

Effect of heating rate on the crystal composition of ferroelectric lithium niobate crystallites

A series of lithium niobate powders were synthesized by the combustion method at different heating rates. The effect of heating rate on the crystal composition of lithium niobate powders was investigated by powder X-ray diffraction measurements. It has been found that the lithium content in the as-synthesized lithium niobate powders increases with decreasing the heating rate. On the basis of the existed structure-property relationship of lithium niobate single crystals, it was concluded that high quality lithium niobate powders can be effectively synthesized at a lower heating rate (in the range of 5-10 C/min) by the combustion method.

ESR Study of a Cr5+ Centre in Ferroelectric Ammonium Sulphate

X-band electron spin resonance (ESR) studies of (CrO4)2- doped, X-irradiated single crystals of ferroelectric ammonium sulphate ((NH4)2SO4, TC = 223 K) at 300 and 208 K are reported. The paramagnetic centre responsible for the ESR spectrum is identified to be Cr5+. Superhyperfine interaction of the unpaired electron with two equivalent protons is observed. The spin-Hamiltonian parameters which are nearly axial at 300 K, with g < g indicating a dx2-y2 orbital ground state, acquired rhombic character below TC indicating a distortion of the sulphate tetrahedron. An increase in the value of the proton superhyperfine constant in the ferroelectric phase is indicative of stronger hydrogen bonding.

The change of complete symmetry of the crystals during the phase transitions in ferroelectrics and ferromagnetics

The extension of the superposition principle of the symmetries (P. Curie principle of symmetry) for the case of complete symmetry is given. The enumeration of all crystallographical groups of complete symmetry is presented, the number of elements having complete symmetry for each class of the crystals being indicated. The change of complete symmetry of the crystals under the phase transitions is obtained by superimposing the elements of complete symmetry of polar or axial vectors on the one hand, and the elements of complete symmetry of the crystals on the other. The tables of complete symmetry changes for the cubic, rhombic, monoclinic and triclinic crystals during the ferroelectric and ferromagnetic phase transitions are given.