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 exhi bits a low and a high frequency absorption peak (PL, PH) which have be en studied as a function of temperature and bias electric field. Measu rements from 10Hz to 10MHz were carried out with smectic layers parall el and quasi-perpendicular to the cell plates for the multicomponent m ixture CS-4000 (Chisso). In addition to the orthogonal smectic A phas e, this material has four tilted phases, three narrow phases with a di electric 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 proce sses first increases by about a factor of two, then exhibits a maximum at a threshold field E(C) corresponding to the antiferroelectric-ferr oelectric transition at which it decreases by almost one order of magn itude. In fact, at E(C) the P-H peak vanishes and the P-L peak shows u p 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 depend ence for E < E(C). We attribute the P-H process to the collective reor ientation of the molecules around the cone in the opposite direction ( anti-phase in the phi variable). P-L may be attributed to a similar co llective reorientation in the same direction (in-phase) around the con e, where the coupling to the electric field is mediated by the helical superstructure, and a corresponding small shift in the local polariza tion directions. There is no antiferroelectric soft mode coupling to a n electric held, but the anti-phase cone motion acts electro-optically in a way similar to the electroclinic effect.