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 SC_A* to ferroelectric SC*, has been emphasised and the antiferroelectric herringbone structure of SC_A* has been presented. Then we have explained how to identify the subphases in the SC* region, e.g. SC_γ*, SC_α*; the clarification of the subphase structures is essential for understanding antiferroelectricity in liquid crystals. After summarizing the evidence for the SC_A* 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 SC_A* 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_α* phase are its considerably reduced ability to form SC_A* and SC*. At least when the spontaneous polarization is zero, SC_α* 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_α*; the competition between this antiferroelectricity and the SC* ferroelectricity may form another staircase, causing the complexity in SC_α*. Applications and some future problems have been described in the final section.

Any polar-ordered material with a spatially uniform polarization field is internally frustrated: The symmetry-required local preference for polarization is to be nonuniform, i.e., to be locally bouquet-like or “splayed.” However, it is impossible to achieve splay of a preferred sign everywhere in space unless appropriate defects are introduced into the field. Typically, in materials like ferroelectric crystals or liquid crystals, such defects are not thermally stable, so that the local preference is globally frustrated and the polarization field remains uniform. Here, we report a class of fluid polar smectic liquid crystals in which local splay prevails in the form of periodic supermolecular-scale polarization modulation stripes coupled to layer undulation waves. The polar domains are locally chiral, and organized into patterns of alternating handedness and polarity. The fluid-layer undulations enable an extraordinary menagerie of filament and planar structures that identify such phases.

Helical structures were confirmed for both the SmA _b and SmBlue phases of banana-shaped molecular systems from observations of the microscopic fringe pattern and the selective reflection of blue color, respectively. X-ray and optical microscopy indicate that the helical axes in the SmA _b and SmBlue phases are normal and parallel to the smectic layer, respectively. In these two helical phases, ¹³C NMR spectra show two C=O peaks, suggesting two different configurations of ester group, whereas only one C=O peak appears in the isotropic and crystal phases. This indicates that the two C=O groups in the mesogenic core are not in the same plane but are twisted. The addition of chiral dopant makes the dichroic ratio of the right- and left-circularly polarized scattered light positive or negative in the SmBlue phase, although the wavelength (∼430 nm) of the scattering peak does not change significantly. The origin of the helix will be discussed in view of the twisted molecular conformation (conformational chirality) and the escape from macroscopic polarization.