Thermodynamic model for the description of smectic C* and smectic A–smectic C* phase transition properties

Abstract

We construct a phenomenological model to describe the general features of chiral smectic C* (Sm C*) phase and Sm A–Sm C* phase transition properties of ferroelectric liquid crystals. 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. Since Q _ij describes the tilt vector ξ, no term contains ξ. The values of Landau coefficients are determined by comparing with the experimental data of helical pitch, tilt angle and spontaneous polarization and introducing an empirical relation for the temperature variation of the modulus S of orientational order parameter Q _ij . Taking these values we evaluate the tilt angle, spontaneous polarization and pitch of the Sm C* phase and the Sm A–Sm C* phase transition. It is found that the theoretical results agree very well with the experimental data of four ferroic mesogens.

Keywords:

• ferroelectric liquid crystals
• phase transitions
• thermodynamic model
• Landau free-energy density

Acknowledgements

We are grateful to Profs Tom C. Lubensky, Harald Pleiner and N.V. Madhusudana for their suggestions during the course of this work. The financial support received from the Department of Science and Technology (DST), Government of India, New Delhi is gratefully acknowledged.