The phase transitions in an antiferroelectric liquid crystal((R)-MHPOBC) and a ferroelectric liquid crystal(CS1017) were studied by polarization reversal, D--E hysteresis loop and dielectric measurements.
Dielectric constant near the $SmA-SmC^*$ phase transition showed the Curie-Weiss behaviour in the ferroelectric liquid crystal CS1017. Slowing down of dielectric relaxation toward the $SmA-SmC^*$ phase transition, associated with the soft mode, was observed in CS1017.
Dielectric measurements as a function of both d.c.bias field and a.c.probe frequency were made in the $SmC^*_A$ phase of (R)-MHPOBC compound to study the field induced phase trnsition from the antiferroelectric to the ferroelectric phase. Three dielectric relaxation peaks observed in the concerned region of the field induced transition were analyzed in terms of the Cole-Cole equations and the Landau free energy functional theory of Orihara and Ishibashi to derive the dielectric relaxation frequencies and the threshold electric fields associated with the transition.
D-E hysteresis loops were studied at various driving frequencies from 1 Hz to 5 KHz for a 3㎛ thick sample of (R)-MHPOBC at T $\simeg$ 90℃. A double-loop hysteresis curve observed in the antiferroelectric phase was transformed into a single loop hysteresis curve with increasing the driving frequency above 500Hz. A relaxation peak was observed at the same frequency in the dielectric loss measurements under d.c.bias fields. Polarization reversal current was also measured to obtain the field induced polarization as a function of the field strength. All of these measurements confirm that the field induced phase transition undergoes from the antiferroelectric to the ferroelectric phase.
A phenomenological model is elaborated for a microscopic understanding of the phase transition from antiferroelectric to ferroelectric phase under an external electric field. The antiferroelectric phase, stabilized by the two layer pairing interactions, was found to become unstable as the electric field is increased above a threshold value $E_c=2JP_o$, where J is the nearest neighbor interaction constant of the molecular dipoles in the adjacent smectic layers and $P_o$ the molecular dipole moment. When the applied electric field brings about the instability of the antiferroelectric phase, the three layer pairing interactions are assumed to stabilize the system in a different configuration possessing ferrielectric ordering. The observed dielectric constant in the antiferroelectric (R)-MHPOBC as a function of the electric field could be understood in the model with the acceptable range of parameters.
