Discovery of a novel ferrielectric phase of five-layer periodicity in binary mixtures of chiral smectic liquid crystals exhibiting unusual reversed phase sequence (original) (raw)
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Physical Review E, 1998
Using a high resolution optical polarimeter, we have measured the temperature dependence of the birefringence and optical rotation in chiral smectic liquid crystals that exhibit antiferroelectric, ferroelectric, and intermediate phases. The temperature dependence of the magnitude of the tilt angle was determined from the birefringence of 4-͑1-methyl-heptyloxycarbonyl-phenyl͒ 4Ј-octylbiphenyl-4-carboxylate ͑MHPOBC͒ and of 4-͑1-methylheptyloxycarbonyl-phenyl͒ 4Ј-octylcarbonyloxybiphenyl-4-carboxylate ͑MHPOCBC͒. Both substances exhibit a crossover of the order parameter exponent from the classical value of ϭ0.5 close to the transition to the tricritical value ϭ0.25 far away. This stresses the importance of the sixth order terms in the Landau free-energy expansion for ferroelectric and antiferroelectric liquid crystals. In addition, a discontinuous behavior in the magnitude of the tilt is observed when crossing the smectic-C ␣ * -smectic-C* or smectic-C* -smectic-C A * transitions, whereas the smectic-A -smectic-C ␣ * transition is continuous. The simultaneously determined optical rotation is used to elucidate the structures and the nature of phase transitions. The results are well explained within the framework of a discrete phenomenological model with nearest and next nearest neighbor interactions between the smectic layers.
XIV Conference on Liquid Crystals: Chemistry, Physics, and Applications, 2002
Two binary mixtures have been studied of homologues from a series derived from 4-alkoxybiphenyl-4'-carboxylic acid with three chiral centres exhibiting ferroelectric (FE), antiferroelectric (AF) and tilted hexatic phases. One homologue exhibits a re-entrant SmC* (SmC* re) phase, which is miscible with the SmC* phase. The phase diagrams of the studied mixtures exhibit a close pocket of the SmC* A phase. The SmC* re phase is induced even in the mixtures composed of homologues without this phase. Dielectric spectroscopy reveals the Goldstone mode in both ferroelectric SmC* and the SmC* re phases, this mode being stronger when increasing concentration of the antiferroelectric homologue. In the SmC* A phase a relaxation mode is detected with the relaxation frequency three orders higher, which exhibits critical slowing down when approaching the SmC* re phase. This mode, which can be attributed to the anti-phase mode, is responsible for the appearance of the SmC* re phase. In mixtures exhibiting the SmC* A phase a helix twist inversion occurs in the vicinity of the low temperature border of the AF phase.
Physical Review E, 2005
The subphase for the temperature range that lies in between Sm-C A * and the three-layer Sm-C A * ͑1/3͒ subphase has been confirmed to exist using the measurements of electric-field-induced birefringence, optical rotation, and the characteristic reflection bands in the antiferroelectric liquid-crystalline compound, 1-trifluoromethylundecyl-4-͑4Ј-dodecyloxybiphenyl-4-yl-carbonyloxy͒-3-fluorobenzoate ͑12BIMF10͒. The measurements of electric-field-induced birefringence and optical rotatory power are made on 25-m-thick homeotropic cells, and the characteristic reflection bands are observed in free-standing films of thicknesses ranging from 30 to 50 m. Several binary mixtures have been prepared by mixing ͑S͒-12BIMF10 with ͑S͒-4-͑1-methylheptyloxycarbonyl͒-phenyl-4Ј-octylbiphenyl-4-carboxylate ͑MHPBC͒, and the effect of racemization of the compound on the character of the biaxial subphase ͑other than three and four layers͒ is also discussed. The results are interpreted in terms of the Emelyanenko-Osipov model ͓Phys. Rev. E 68, 051703 ͑2003͔͒; the effective long-range couplings between the director orientations in separated smectic layers emerge after the minimization of free energy with respect to the total ͑ordinary spontaneous and discrete flexoelectric͒ polarizations, lifting the degeneracy and producing the nonplanar structures of the subphases.
Ferroelectric ordering and electroclinic effect in chiral smectic liquid crystals
Physical Review E, 2004
Ferroelectric ordering, the electroclinic effect and chiral smectic C (SmC*) -smectic A (SmA*) phase transitions in thin planar ferroelectric liquid crystal (FLC) cells are studied by means of linear electrooptic and second harmonic generation techniques. The ferroelectric switching is detected in biased FLC cells by measuring azimuthal dependences of linear and nonlinear responses. The applied DC-electric field rotates the FLC symmetry axis with initial and final orientations in the cell plane. Comparative studies of the switching behavior in reflection and transmission allows to distinguish the contributions from the bulk and the sub-surface layers of the cell. The analysis of temperature dependence shows the existence of a strong surface coupling. The temperature dependent nonlinear polarization shows a critical behavior corresponding to the superfluid model.
Physical Review E, 2004
Temperature dependences of the infrared absorbance have been measured for the four chiral liquid crystal samples in the homeotropic cell configuration. It is shown that the values of the orientational order parameter S obtained using this method exhibit a remarkable similarity to the x-ray diffraction results of the smectic layer spacing and lead to accurate values of the molecular tilt angle. This has important consequences for the existing interpretation of the x-ray data. The proposed method, in many cases, may be considered as a valuable alternative to the x-ray diffraction, giving additional, important information about the orientations and the ordering of the molecular fragments. It is found that if the experimentally obtained order parameter is low, then the molecular biaxiality is exceptionally large. The average phenyl ring plane is found to lie close to the molecular tilt plane.
Phase transitions in ferroelectric liquid crystals
Phase Transitions, 2008
Phase transitions in thin (1.5-3 lm) planar cells of a ferroelectric liquid crystal (FLC) mixture are studied using the optical second harmonic generation (SHG) technique. This method is well known because of its high sensitivity to the structural and electronic properties of surfaces, interfaces and thin films, as well as to the appearance of the ferroelectric state of a material. It is shown that the studied FLC mixture retains a highly regular structure up to about 80°C, when a sharp decrease of SHG intensity, associated with the transition of the LC to the nonpolar (N*) phase, is observed. SHG intensity measurements show that the transition from chiral smectic C (SmC*) to chiral smectic A (SmA*) phase which takes place at about 45°C for the bulk FLC mixture does not influence the ferroelectric state of subsurface FLC layers. At the same time, SHG interferometry measurements show that definite changes in the FLC structure happen above the bulk transition temperature from SmC* to SmA* phase. A DC electric field induced switching of the FLC cells is also observed. Ó
Journal of Thermal Analysis and Calorimetry, 2005
Thermal properties of a homologous series of ferroelectric liquid crystals S-(-)- [4-(2-n-alkoxy-propionyloxy)] biphenyl-4'-[n-alkoxy-(3,5-dimethyl)]benzoate have been investigated by polarizing optical microscopy and differential scanning calorimetry. The mesophases were identified and confirmed by X-ray too. Three binary mixtures were prepared from the individual homologues. In one of the mixtures (Mix1), the ferroelectric SmC* phase has broadened and became enantiotropic. This mesophase remained monotropic in the other two mixtures (Mix2, Mix3). The chiral nematic N* phase did not appear in Mix1, but remained monotropic for the other two mixtures. Two molecular parameters, the layer spacing and the average intermolecular distance have been calculated from the X-ray results for the homologues and their mixtures. An intercalated tail-to-tail packing of molecules was found both in the single compounds and their mixtures resulting in the layer spacing about half of the molecular length of the single compounds.
Electric field induced phase transition in a ferroelectric smectic-C* liquid crystal
The European Physical Journal Applied Physics, 2011
We have experimentally investigated the unwinding process of the ferroelectric Sm-C * phase of a chiral AFLC smectic liquid crystal compound 12OF1M7 using the electro-optic properties, microscopic observations and constant current method. We found that this process occurs via two steps through an intermediate phase. Furthermore, we have established the (E-T) phase diagram of the studied compound by combining the experimental results.
Physical Review Letters, 2001
The free energy of antiferroelectric liquid crystal which takes into account polar order explicitly is presented. Steric, van der Waals, piezoelectric and flexoelectric interactions to the nearest layers and dipolar electrostatic interactions to the nearest and to the next nearest layers induce indirect tilt interactions with chiral and achiral properties, which extend to the third and to the fourth nearest layers. Chiral indirect interactions between tilts can be large and induce helicoidal modulations even in systems with negligible chiral van der Waals interactions. If indirect chiral interactions compete with chiral van der Waals interactions, the helix unwinding is possible. Although strength of microscopic interactions change monotonically with decreasing temperature, effective interlayer interactions change nonmonotonically and give rise to nonmonotouous change of modulation period through various phases. Increased enatiomeric excess i.e. increased chirality changes the phase sequence.
Spectroscopy Letters, 2008
Structural, electro-optical, and dielectric investigations of a ferroelectric liquid crystal (FLC) exhibiting the chiral smectic C phase (SmC Ã ) are reported. This study has been performed on the pure FLC showing the SmC Ã -SmA-N Ã phase sequence and having high spontaneous polarization and a large relaxation frequency. We have determined the Goldstone rotational viscosity and the twist elastic constant in the SmC Ã phase from the experimental data of helical pitch, tilt angle, polarization, dielectric strength, and relaxation frequency of the Goldstone-mode relaxation. An Arrhenius-type behavior of the Goldstone rotational viscosity was obtained, and the corresponding activation energies were evaluated.