Refractive index measurements at a 2nd order smectic A to C phase transition

A measurement of the 3 principal refractive indices of azoxy-4,4' (di-undecyl α-methyl cinnamate) is made in the vicinity of the 2nd order smectic-A to smectic-C phase transition. The experimental procedure is : first the index differences are measured using the conoscopic method, then the absolute value of one of the indices is determined using an Abbe refractometer. In the smectic-A phase, a slow quasi-linear temperature dependence of the indices is found. In the smectic-C phase a faster effect adds to the smectic-A background. This effect is studied in more detail, and discussed in terms of angular fluctuations of the molecules.