Measurement of refractive indices and their relation to orientational order at smectic-A—smectic-Ctransitions

We report the measurement of the principal indices of refraction, and the density as a function of temperature for five liquid crystals from two homologous series. These data are used to calculate the corresponding polarizabilities after first examining published models for calculating the internal fields. The Neugebauer model, which allows for anisotropic internal fields, was chosen as the basis for interpreting data. The biaxiality of the smectic-$C$ phase is shown to be small, and possible interpretations using the Straley order parameters are discussed. The effects of varying the amount of smectic order at the $A-C$ transition are also discussed. Finally the indices of refraction in the smectic-$C$ phase are used to obtain the value of the optic angle versus temperature. Crudely, this temperature dependence goes as ${(T_{\mathrm{AC}}-T)}^{0.27}$ which seems to favor theoretical models in which the molecule rotates freely in the smectic-$C$ phase.