THE MOLECULAR ASPECT OF THE DOUBLE ABSORPTION PEAK IN THE DIELECTRIC SPECTRUM OF THE ANTIFERROELECTRIC LIQUID-CRYSTAL PHASE

The dielectric spectrum of the antiferroelectric smectic Cb phase exhibits a low and a high frequency absorption peak (PL, PH) which have been studied as a function of temperature and bias electric field. Measurements from 10Hz to 10MHz were carried out with smectic layers parallel and quasi-perpendicular to the cell plates for the multicomponent mixture CS-4000 (Chisso). In addition to the orthogonal smectic A* phase, this material has four tilted phases, three narrow phases with a dielectric behaviour permitting us to classify two of them as C*(alpha) (82.800 to 81.98 degrees C) and C*(gamma) (80.100 to 79.17 degrees C), and one broad antiferroelectric phase (79.17 degrees to - 10 degrees C). On applying an increasing bias field, Delta epsilon for both processes first increases by about a factor of two, then exhibits a maximum at a threshold field E(C) corresponding to the antiferroelectric-ferroelectric transition at which it decreases by almost one order of magnitude. In fact, at E(C) the P-H peak vanishes and the P-L peak shows up at a frequency slightly lower than that corresponding to zero field. In contrast to the Delta epsilon behaviour, the relaxation frequency of the two absorptions does not show any appreciable bias field dependence for E < E(C). We attribute the P-H process to the collective reorientation of the molecules around the cone in the opposite direction (anti-phase in the phi variable). P-L may be attributed to a similar collective reorientation in the same direction (in-phase) around the cone, where the coupling to the electric field is mediated by the helical superstructure, and a corresponding small shift in the local polarization directions. There is no antiferroelectric soft mode coupling to an electric field, but the anti-phase cone motion acts electro-optically in a way similar to the electroclinic effect.