Landau theory description of polar smectic structures

In this paper we review various amazing features of polar smectic structures and phase transitions between them. In a series of experimental and theoretical studies over the last decades, a lot of new and unexpected structures have been observed and described theoretically in the realm of polar smectic liquid crystals. Not all of these structures were correctly identified experimentally and/or correctly interpreted theoretically. At least in part, the problem is in the uncertainty about which theoretical model can provide an adequate minimal description of all available experimental data. The approach we are advocating in this work is to rely on a systematic, well elaborated, and perfectly suitable to describe mean field behavior in the vicinity of phase transition lines, theoretical frame – Landau phase transitions theory. Structural and symmetry properties of the polar smectic liquid crystals require utilization of the Landau theory with the discretized over the smectic layers two-component order parameter. Furthermore, both components of the order parameter (its modulus, related to interlayer distance, and phase, related to azimuthal orientation of molecules) can be space non-uniform, describing various smectic structures. We show that this theory describes the experimentally discovered polar smectic phases and predicts a number of new structures.