Effect of molecular rotational hindrance on the spontaneous polarization in the S*C phase of a ferroelectric liquid crystal

The spontaneous polarization, P_s, and the tilt angle, θ, have been measured simultaneously in the S*_C phase of binary mixtures of two ferroelectric liquid crystals as a function of temperature and concentration. A molecular rotational model was used in order to calculate the temperature dependence of the P_s/θ ratio. A two parameter fit of the experimental data was carried out which showed good agreement. The molecular rotational potentials have been determined from the fit parameters and are presented as a function of concentration. It is shown that the chiral part of the potential depends very weakly on the composition in contrast to the non-chiral part which shows a strong negative non-linear deviation with a minimum corresponding to the concentration at which the smectic C* phase width is maximal.