Dielectric behavior of ferroelectric liquid crystals in the vicinity of the transition into the hexatic phase

Collective molecular dynamics was studied in ferroelectric smectic C-* and hexatic smectic I-* phases by the dielectric method. In the frequency range 1 Hz to 10 MHz one relaxation process was observed in both phases. In the SmC* phase the relaxation frequency of this process linearly decreases and its dielectric strength increases when approaching the SmC*-SmI* phase tran sition temperature T-I. In the SmI* phase the relaxation frequency is one o rder of magnitude lower and further slightly decreases on cooling. A phenom enological theory has been developed which describes static and dynamic die lectric properties near T-I and satisfactorily explains the experimental re sults. The observed relaxation has been attributed to fluctuations of the b ond orientational order coupled to the molecular tilt. A parameter describi ng the softening of this mode is determined by fitting theoretical results to the experimental data. The model also describes an observed anomalous in crease in the spontaneous polarization as a result of the change in the str ength of bond orientational order when passing to the SmI* phase. (C) 1999 American Institute of Physics. [S0021-9606(99)51928-4].