Transformation of polar nematic phases in the presence of an electric field

Only a few years have passed since the 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 take a step toward systematization. The powerful technique of molecular-statistical physics has been applied to an assembly of polar molecules influenced by electric field. 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 of or at low electric field), the double-splay antiferroelectric nematic $N_{AF}$ (observed at intermediate temperature in the absence of 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 paradoxical 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 the application of electric field has also been investigated by Monte Carlo simulations and experimentally by observation of polarizing optical microscope images. In particular, it has been realized that, at planar anchoring, $N_{AF}$ in the presence of a moderate out-of-plane electric field exhibits twofold splay modulation: antiferroelectric in the plane of the substrate and ferroelectric in the plane normal to the substrate. Several additional subtransitions related to fitting the confined geometry of the cell by the structure of polar phases were detected.