Deduction of the temperature-dependent structure of the four-layer intermediate smectic phase using resonant X-ray scattering

Chiral Symmetry Breaking in Liquid Crystals: Appearance of Ferroelectricity and Antiferroelectricity

The study of chiral symmetry breaking in liquid crystals and the consequent emergence of ferroelectric and antiferroelectric phases is described. Furthermore, we show that the frustration between two phases induces a variety of structural phases called subphases and that resonant X-ray scattering is a powerful tool for the structural analysis of these complicated subphases. Finally, we discuss the future prospects for clarifying the origin of such successive phase transition.

Transitional subphases near the electric-field-induced phase transition to the ferroelectric phase in Se-containing chiral smectic liquid crystals observed by resonant x-ray scattering

Resonant x-ray scattering experiments revealed transitional subphases near the electric-field-induced phase transition of a Se-containing chiral liquid crystal in a planar aligned cell geometry. In the lower-temperature range (Sm-C_{A}^{*} and three-layer periodicity Sm-C_{γ}^{*} phases), the six-layer periodicity subphase appeared with increasing electric field during the field-induced transition from Sm-C_{γ}^{*} to Sm-C^{*}. In the higher-temperature range [four-layer periodicity antiferroelectric (AF) phase], the peak positions of the three-layer satellites shifted to those of the four-layer satellites and then the satellites corresponding to the five- through seven-layer periodicity appeared in sequence. Near the AF to Sm-C_{α}^{*} phase transition temperature, the layer periodicity increased with applied field. The molecular configurations of the subphases near the field-induced transition are discussed based on the Ising, distorted clock, and perfect clock models.

Effective long-range interlayer interactions and electric-field-induced subphases in ferrielectric liquid crystals

Microbeam resonant x-ray scattering experiments recently revealed the sequential emergence of electric-field-induced subphases (stable states) with exceptionally large unit cells consisting of 12 and 15 smectic layers. We explain the emergence of the field-induced subphases by the quasimolecular model based on the Emelyanenko-Osipov long-range interlayer interactions (LRILIs) together with our primitive way of understanding the frustration in clinicity using the q_{E} number defined as q_{E}=|[R]-[L]|/([R]+[L]); here [R] and [L] refer to the numbers of smectic layers with directors tilted to the right and to the left, respectively, in the unit cell of a field-induced subphase. We show that the model actually stabilizes the field-induced subphases with characteristic composite unit cells consisting of several blocks, each of which is originally a ferrielectric three-layer unit cell stabilized by the LRILIs, but some of which would be modified to become ferroelectric by an applied electric field. In a similar line of thought, we also try to understand the puzzling electric-field-induced birefringence data in terms of the LRILIs.

Chiral smectic transition phases appearing near the electric-field-induced phase transition observed by resonant microbeam x-ray scattering

The electric-field-induced phase transition of a chiral liquid crystal containing Br revealed a transition phase between the three-layer periodicity ferrielectric phase and the synclinic ferroelectric phase in the electric field versus temperature phase diagram. Resonant x-ray scattering from the transition phase showed a diffuse streak or spotty weak reflections, which were composed of strong m/3-order (where m = 1 and 2) reflections and other weak peaks. The spotty reflections were found to be related to a 12-layer periodicity phase with a weak contribution from the 15-layer periodicity. An x-ray intensity analysis based on the Ising model suggested that the 12-layer periodicity phase was composed of two three-layer ferrielectric blocks and six synclinic layers. This model indicated that, in the transition phase, the three-layer ferrielectric molecular configuration gradually changed to the synclinic configuration. The diffuse streak appearing around m/3-order reflections near the field-induced transition from the four-layer periodicity phase to the synclinic ferroelectric phase is also discussed.

Microbeam resonant x-ray scattering from bromine-substituted bent-core liquid crystals

We studied the local layer structure in the B2 phase of bromine-substituted bent-core liquid crystals in the cell geometry using microbeam resonant x-ray scattering. In the homochiral state of B2 phase, the 3/2 order satellite peak was observed only when the incident x-ray energy is at the K absorption edge of bromine. This result clearly indicates that the B2 homochiral domain forms two-layer superlattice in adjacent layers, the same as in the rodlike Sm-C(A) phase. The work reports on microbeam resonant x-ray scattering experiment from the local layer of the bent-core liquid crystal in the device geometry. Moreover, we can say that bromine is also useful for the analysis of the superstructure of soft materials using resonant x-ray scattering.

Stabilization of the smectic-Calpha* phase in mixtures with chiral dopants

A series of mixtures comprising an antiferroelectric liquid-crystal host and a chiral dopant is described in which the layer spacing variation at the orthogonal smectic-A* (SmA*) to tilted smectic-C* or smectic-Calpha* (SmC* or SmCalpha*) phase transition changes from the usual strong contraction in the pure system to one in which there is almost no layer spacing change observed across the transition for dopant concentrations of 7%. The nature of the orthogonal to tilted phase transition is examined using Raman spectroscopy, to determine the order parameters <P2> and <P4> in the SmA* phase, and via a generalized Landau expansion to reveal the details of the phase transition itself. The results show that the value of <P2> at the orthogonal to tilted transition increases from around 0.6 to 0.7 as the dopant concentration increases, while <P4> remains constant at approximately 0.4 irrespective of dopant concentration. Further, the generalized Landau potential measurements prove that the transition is purely second order, while electro-optic measurements confirm that the tilt angle at the transition becomes smaller with increasing dopant concentration. The combined data show that the high-temperature tilted phase regime corresponds to a SmCalpha* phase rather than the mechanism suggested by de Vries that is inferred by the layer spacing data alone. We demonstrate that the lower-temperature SmCalpha*-SmC* phase transition is of first order. Further, the temperature range of the SmCalpha* phase increases dramatically with concentration, from around 2 K in the pure system to around 21 K in the 8% doped mixture, showing that the chiral dopant plays a role in stabilizing this phase. Indeed, we particularly note that for the 8% doped mixture all other SmC*-like phases disappear and that the only tilted phase remaining is SmCalpha*. This implies that we are reporting a liquid-crystalline phase sequence, namely, cryst.-SmCalpha*-SmA*-iso., i.e., a direct transition between the SmCalpha* phase and the crystalline phase.

Unexpected field-induced phase transitions between ferrielectric and antiferroelectric liquid crystal structures

Liquid crystals are intriguing electrically responsive soft matter systems. We report previously unexplored field-induced changes in the structures of some frustrated liquid crystal phases and describe them theoretically. Specifically, we have discovered using resonant x-ray scattering that the four-layer intermediate smectic phase can undergo either a transition to the ferrielectric (three-layer) phase or to the ferroelectric phase, depending on temperature. Our studies of intermediate phases using electric fields offer a way to test theories that describe ferroelectricity in self-assembling fluids.