Transformation of polar nematic phases in the presence of electric field
Only a few years have passed since discovery of polar nematics, and now they are becoming the most actively studied liquid crystal materials. Despite numerous breakthrough findings made recently, a theoretical systematization is still lacking. In the present paper we are making a step on the way of systematization. A powerful technique that molecular-statistical physics is has been applied to an assembly of polar molecules influenced by electric field. Totally, the three polar nematic phases were found to be stable at various conditions: the double-splay ferroelectric nematic $N_F^{2D}$ (observed in the lower-temperature range in the absence or at low electric field), the double-splay antiferroelectric nematic $N_{AF}$ (observed at intermediate temperature in the absence or at low electric field) and the single-splay ferroelectric nematic $N_F^{1D}$ (observed at moderate electric field at any temperature below transition into paraelectric nematic $N$ and in the higher-temperature range (also below $N$) at low electric field or without it. A paradoxal transition from $N_F^{1D}$ to $N$ induced by application of higher electric field has been found and explained. A transformation of the structure of polar nematic phases at application of electric field has also been investigated by Monte Carlo simulations and experimentally by observation of POM images. In particular, it has been realized that, at planar anchoring, $N_{AF}$ in the presence of moderate out-of-plane electric field exhibits the twofold splay modulation: antiferroelectric in the plane of the substrate and ferroelectric in the plane normal to the substrate. Several additional sub-transitions related to fitting confined geometry of the cell by the structure of polar phases were detected.