Temperature Dependence of the Layer Spacing in the Smectic Z_A Phase of Ferroelectric Nematic Liquid Crystals

 

The antiferroelectric, smectic-Z_A phase occurs as a function of temperature between non-polar and ferroelectric nematic (N_F) phases in several ferroelectric nematic compounds and mixtures. It is characterized by a weak mass density wave (period d_M) that accompanies an antiferroelectric polarization field, which alternates with period 2d_M in the same direction as the density wave. Here we report on small-angle X-ray scattering (SAXS) studies, performed on a laboratory SAXS/WAXS system, that contrast the temperature dependence of d_M in two ferroelectric nematic materials, FNLC919 and DIO. In FNLC919, we observe a pronounced pretransitional increase in d_M on approach to the N_F phase, whereas DIO exhibits a much weaker temperature dependence. We compare these results to predictions of certain theoretical models for the smectic-Z_A phase and smectic-Z_A to N_F transition.