Antiferroelectric chiral smectic liquid crystals

Abstract

In a brief history of the discovery of antiferroelectricity in liquid crystals, the important role played by tristable switching, i.e. an electric field induced phase transition from antiferroelectric SCA* to ferroelectric SC*, has been emphasised and the antiferroelectric herringbone structure of SCA* has been presented. Then we have explained how to identify the subphases in the SC* region, e.g. SC,*, SC,*; the clarification of the subphase structures is essential for understanding antiferroelectricity in liquid crystals. After summarizing the evidence for the SCA* structure presented, we have suggested the pair formation of transverse dipole moments in adjacent smectic layers as the cause of its antiferroelectricity, showing that the smectic layer is much closer to the usual picture of molecules lying on equidistant planes; the packing entropy due to the sinusoidal density wave character stabilizes ferroelectric SC*. The competition between the interactions stabilizing SCA* and SC* is responsible for the occurrence of several varieties of ferrielectric and antiferroelectric subphases, which constitutes the Devil's staircase. We have further suggested that the essentials of the SC,* phase are its considerably reduced ability to form SCA* and SC*. At least when the spontaneous polarization is zero, SC,* is a smectic C-like phase with molecular tilting that is non-correlated on the visible wavelength scale When the spontaneous polarization is not zero, as suggested by Prost and Bruinsma recently, a novel type of Coulomb interaction between smectic layers due to the collective polarization fluctuations causes the antiferroelectricity in the high-temperature region of SC,*; the competition between this antiferroelectricity and the SC* ferroelectricity niay form another staircase, causing the complexity in SC;. Applications and some future problems have been described in the final section.

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

“…14 (b) with a relatively large value of δ. At 84.5 °C, in contrast to the results at 83.9 °C, neither clear C 3p phase nor the C 5p subphase were confirmed, though very weak peaks at q/q o = 1/3 (or 2/6) were observed at ±27.6 V. At 83.9 °C and at 84.5 °C, the characteristic sequential transition, i.e., AF → SmC * γ (C 3p ) → SmC * [3,29], takes place and the temperature 84.5 °C is close to the critical point where the C 3p phase disappears in the E-T diagram. The novel C 4p2 subphase and other transitional subphases at 84.5 °C may be related to this complicated situation.…”

“…In contrast, the previously reported C 6p layer structures provided the strong 1/6 and 5/6 reflections [22,25]. The Ising (flat), distorted clock, and (perfect) clock models [3] were examined to analyze the molecular arrangement of this new C 6p…”

“…Figs. 12 (a) to ( in a unit cell [3,31], is a reasonable layer structure of the subphase appearing between the C 3p (q E = 1/3) and Sm C * (q E = 1) phases. From these considerations, the C 6p subphase observed here is close to the {R 5 L} configuration with a relatively small value of δ, however, the more complicated distorted C 6p structures cannot be ignored.…”

“…antiferroelectric AF (SmC * FI2 , SmC * d4 , or SmC * A (q T = 1/2)), ferrielectric SmC * γ (SmC * FI1 , SmC * d3 , or and other techniques [3,6,[27][28][29][30][31][32]. Recent RXS studies under an electric field directly revealed the electric field-induced phase transitions from the low field AF phase to the high field SmC * γ phases [33][34][35].…”

“…5 at the low applied field (±28 V and ±29 V). However, the optical biaxiality [3] and the distorted clock structure [13] were reported for the C 3p phase;…”