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We report the properties of two new series of compounds that show the ferroelectric nematic phase in which the length of a terminal chain is varied. The longer the terminal chain, the weaker the dipole-dipole interactions of the molecules are along the director, and thus the lower the temperature at which the axially ferroelectric nematic phase is formed. For homologues of intermediate chain length, between the non-polar and ferroelectric nematic phases, there is a wide temperature range nematic phase with antiferroelectric character. The size of the antiparallel ferroelectric domains critically increases upon transition to the ferroelectric phase. In dielectric studies, both collective ("ferroelectric" and non-collective fluctuations are present, the "ferroelectric" mode softens weakly at the N-NX phase transition because the polar order in this phase is weak. The transition to the NF phase is characterized by a much stronger lowering of the mode relaxation frequency and an increase in its strength, typical critical behavior is observed.

A critical end point above which an isotropic phase continuously evolves into a polar (ferroelectric) nematic phase with an increasing electric field is found in a ferroelectric nematic liquid crystalline material. The critical end point is approximately 30 K above the zero-field transition temperature from the isotropic to nematic phase and at an electric field of the order of 10 V/micron. Such systems are interesting from the application point of view because a strong birefringence can be induced in a broad temperature range in an optically isotropic phase.

Elastic moduli of liquid crystals, known as Frank constants, are of quintessential importance for understanding fundamental properties of these materials and for the design of their applications. Although there are many methods to measure the Frank constants in the nematic phase, little is known about the elastic constants of the chiral version of the nematic, the so-called cholesteric liquid crystal, since the helicoidal structure of the cholesteric renders these methods inadequate. Here we present a technique to measure the bend modulus $K_{33}$ of cholesterics that is based on the electrically tunable reflection of light at an oblique helicoidal $Ch_{OH}$ cholesteric structure. $K_{33}$ is typically smaller than 0.6 pN, showing a non-monotonous temperature dependence with a slight increase near the transition to the twist-bend phase. $K_{33}$ depends strongly on the molecular composition. In particular, chiral mixtures that contain the flexible dimer 1'',7''-bis(4-cyanobiphenyl-4'-yl) heptane (CB7CB) and rod-like molecules such as pentylcyanobiphenyl (5CB) show a $K_{33}$ value that is 5 times smaller than $K_{33}$ of pure CB7CB or of mixtures of CB7CB with chiral dopants. Furthermore, $K_{33}$ in CB11CB doped with a chiral agent is noticeably smaller than $K_{33}$ in a similarly doped CB7CB which is explained by the longer flexible link in CB11CB. The proposed technique allows a direct in-situ determination of how the molecular composition, molecular structure and molecular chirality affect the elastic properties of chiral liquid crystals., Comment: 20 pages, 8 figures