Collective director modes at the transitions to hexatic ferroelectric liquid crystalline phases

Measurements of the dielectric constant and the modulation depth of the light intensity as a function of temperature and electric field frequency are reported for two compounds of the homologous series 4-[(S)-2-chloro-3-methylbutanoyloxy]-4'-[4-n-alkyloxybenzoyloxy]biphenyl exhibiting cholesteric, smectic-A*, smectic-C*, hexatic-I*, and hexatic-F* phases. Relaxation processes related to the hexatic-I*--smectic-C* and hexatic-to-hexatic phase transitions are characterized and the results discussed in terms of bond orientational order and its coupling to the molecular tilt direction. Parameters describing this coupling are determined by fitting the results of a theoretical model to the experimental data.