Dipole ordering in liquid crystals

Abstract

Abstract Liquid crystals are characterized by ordering of molecules, very frequently polar molecules and it is possible to observe, besides the molecular ordering, a dipole ordering. Discussing theoretically the possibility of appearance of such dipole ordering, one can show that it is expected to be observed only in the smectic phases, i.e. phases with a layer structure. This dipole ordering is very likely not ferroelectric, in the sens of a macroscopic polarization (unfortunatly, some times the word ferroelectric is used instead of dipole ordering), but rather of antiferroelectric or helicoidal type. Since the ordering in liquid crystals is essentially due to molecular interaction (liquid crystal phases exist even in compounds with non polar molecules) the dipole ordering is improper and not necessarity commensurate with the molecular ordering in layers. We present the experimental results concerning the two following situations: the helicoidal ordering in the smectic C phase (titled molecules) of compounds made of chiral molecules and the antiferroelectric phase of the double layered smectic A phase of some cyano-compounds. Anomalies has been observed in the dielectric constants of these compounds at the phase transition, and one observes relaxation which is related to the dipole ordering. In the first case, each layer has a true polarization parallel to the layer plane, but the polarization direction varies when passing from layer to layer. One has a helicoidal ordering with a pitch of about 1μ. It is possible to show and to verify that all the dielectric properties are directly related to the helix pitch. It is also possible to transform the sample to a uniform polarized one, by applying an external electric field. In the second case, we have a true antiferroelectric order since we can divide the sample in two sub-lattices with opposite polarization. As in the first case, one observes anomalies in the dielectric constants and a particular relaxation we relate to the antiferroelectric ordering.