BASIC LIQUID-CRYSTAL STRUCTURES, POLYMORPHISMS AND DEFECTS

A review is given of our present general classification of the liquid crystals. The liquid crystal structures are divided into two main groups: nematic phases with parallel ordering of the long molecular axes and smectic phases having additional layered structure. An interesting phenomenon is that the liquid crystal phases when formed by optical active molecules develop chiral modifications: chiral nematics and chiral smectics. There are four fundamental smectic phases: A, B, L and E having skewed analogues: C for smectic A and F, G, H (with long molecular axes tilted to the side of the hexagon) and I, J, K (tilted to the apex of the hexagon) for smectics B, L and E, respectively. Chiral nematics N* and chiral modifications of smectics with weak interlayer correlation (C*, I* and F*) form long-range helicoidal structures. We also briefly discuss the main topological defects in liquid crystal structures: dislocations in layered (smectic) or pseudolayered (chiral nematic) phases and disclinations (including focal domains) that are fundamental defects of structures with continuous symmetries.