The molecular aspect of the double absorption peak in the dielectric spectrum of the antiferroelectric liquid crystal phase

Abstract

The dielectric spectrum of the antiferroelectric smectic C∗ phase exhibits a low and a high frequency absorption peak (P_l, P_h) which have been studied as a function of temperature and bias electric field. Measurements from 10 Hz to 10 MHz 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∗_α (82·80° to 81·91°C) and C∗_γ (80·10° to 79.17°C), and one broad antiferroelectric phase (79.17° to −10°C). On applying an increasing bias field, Δ∊ 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 Δ∊ 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 Ø 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.