Thermodynamic model of ferroelectric liquid crystals and its microscopic basis

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Abstract
The properties of the extended thermodynamic model (Phys. Rev. A 36, 1484 (1987)) of ferroelectric liquid crystals, which can describe the anomalous thermodynamic behaviour of these systems in the chiral smectic C* phase, are reviewed. In the model the bilinear as well as biquadratic coupling between the tilt of molecules and the spontaneous polarization is included. The bilinear coupling is of a chirak nature and therefore small. This leads in the smectic C* phase close to the transition from the smectic A phase to cross-over effects between the regime, where because of small tilt only bilinear coupling is important, to the regime, where the biquadratic coupling also becomes relevant. A microscopic model is introduced, which describes the coupling between the tilt of molecules and the orientation of the molecular dipole, which is perpendicular to the long molecular axis. It is shown that the microscopic model is consistent with the extended thermodynamic model and the parameters of the thermodynamic model are expressed in terms of the microscopic parameters.