Landau model of the smectic C–isotropic phase transition

We propose a Landau model to describe the smectic C–isotropic phase transition. A general Landau theory for the coupled orientational and translational order parameters and including the tilt angle is developed. The conditions for the smectic C–isotropic phase transition and the stability conditions of the smectic C phase are calculated. On the basis of this model it is argued that the smectic C–isotropic phase transition is always first order. We present a detailed analysis of the question under which conditions a direct smectic C–isotropic phase transition prevails in comparison to smectic A–isotropic and nematic–isotropic transitions. The theoretical results are found to be in qualitative agreement with all published experimental results.