DIELECTRIC BIAXIALITY IN SMECTIC C-ASTERISK MATERIALS

The dielectric biaxiality and dielectric anisotropy have been determin ed for helical smectic C materials of varying pitch. The dielectric b iaxiality was found to be positive, the dielectric anisotropy to be ne gative at 100 kHz. Thus, the permittivity component in the direction o f the spontaneous polarisation (C-2 symmetry axis) has the largest val ue among the three principle components of the dielectric tensor, and epsilon(2) > epsilon(1) > epsilon(3). The dielectric biaxiality is fur ther discussed as an order parameter of the tilted smectic phase. The critical behaviour is compared to that of the primary order parameter, the tilt angle, for a number of smectic C compounds. It is verified that if the tilt increases according to theta similar to (T-c - T)(bet a), the biaxiality increases according to delta epsilon similar to (T- c - T)(2 beta).