Mean-field coefficients and the electroclinic effect of a ferroelectric liquid crystal

Experimental data on the temperature and electric field dependence of the tilt angle and the electroclinic effect of the smectic-${\mathit{C}}^{\mathrm{*}}$ (Sm-${\mathit{C}}^{\mathrm{*}}$) and the smectic-${\mathit{A}}^{\mathrm{*}}$ (Sm-${\mathit{A}}^{\mathrm{*}}$) phases are presented which have been obtained by electrooptical investigations on the ferroelectric liquid crystal mixture FLC 6430 from Hoffmann–La Roche, Basle, Switzerland. The data are analyzed according to the generalized Landau expansion and the results of a microscopic model for spontaneous polarization. The mean-field coefficients obtained from a fit of the Landau approach to the experimental data are discussed with respect to the electroclinic effect, the specific heat singularity, and the chirality. The results support the evidence of quadrupolar ordering and biquadratic coupling for the description of the Sm-${\mathit{C}}^{\mathrm{*}}$-Sm-${\mathit{A}}^{\mathrm{*}}$ phase transition. The Landau model is extended to temperatures far below the phase transition.