Thermodynamic model for the electro-optical and structural properties of Sm C*A phase in antiferroelectric liquid crystal MHPOBC

Abstract

We construct a phenomenological model to describe the electro-optical and structural properties of the SmC^*_A phase of antiferroelectric liquid crystal MHPOBC. The free-energy density of the system is expanded in terms of three degrees of freedom—tensor orientational order Q_ij, scalar smectic order Ψ, and polarization vector P and couplings between these order parameters. In this expansion the tilt vector ξ is not incorporated because ξ is governed by Q_ij. The values of Landau coefficients are determined by comparing with the experimental data of helical pitch, tilt angle and spontaneous polarization of MHPOBC in the SmC^*_A phase. Taking these values we evaluate the temperature variation of the tilt angle, spontaneous polarization and pitch of the SmC^*_A phase for MHPOBC. It is found that the theoretical results agree very well with the experimental data for antiferroic mesogens.