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.

在本文中，我们回顾了极性近晶结构的各种惊人特征及其之间的相变。在过去几十年的一系列实验和理论研究中，已经在极性近晶液晶领域中观察到并从理论上描述了许多新颖的结构。并非所有这些结构都可以通过实验正确识别和/或从理论上正确解释。至少部分地，问题在于不确定哪个理论模型可以对所有可用的实验数据提供足够的最小描述。我们在这项工作中提倡的方法是依靠系统的，精心设计的且非常适合于描述相变线附近的平均场行为的理论框架-Landau相变理论。极性近晶型液晶的结构和对称特性需要利用Landau理论，其中近晶型层的二元阶次参数离散。此外，阶数参数的两个分量（其模量（与层间距离有关，而相位与分子的方位角有关））可以是空间不均匀的，描述了各种近晶结构。我们表明，该理论描述了实验发现的极性近晶相并预测了许多新结构。与分子的方位角有关的空间可以是空间不均匀的，描述了各种近晶结构。我们表明，该理论描述了实验发现的极性近晶相并预测了许多新结构。与分子的方位角有关的空间可以是空间不均匀的，描述了各种近晶结构。我们表明，该理论描述了实验发现的极性近晶相并预测了许多新结构。