Novel columnar–calamitic phase sequences in a binary system of bent-core and rod-like mesogens

Abstract

We report here X-ray, electrical switching and dielectric measurements on a binary system comprising achiral bent-core and chiral rod-like components. While the pure bent-core compound exhibits a single mesophase, namely the B2 phase, the rod-like compound shows smectic A and smectic C* mesophases. A particular mixture, studied in detail here, presents a novel sequence of three columnar type B1 phases, smectic A–B1_OPAF1–B1_OPAF2–B1_TPAF–smectic C* phase, as the sample is cooled, where the subscripts O, T, P, F and AF indicate orthogonal, tilted, polar, ferroelectric and antiferroelectric properties, respectively, with the B1 phases being of the B1_rev type. In the electrical switching measurements a twin peak profile – characteristic of an antiferroelectric structure – is seen for the B1_OPAF phases, but with no textural change except on a transient scale. On the other hand the B1_TPAF phase, which also shows antiferroelectric-type switching, exhibits clear changes between the field-off and field-on states, as well as for the two signs of the field. This phase also possesses a three-fold higher value of polarization than the smectic C* phase, indicating the stronger influence of the polar ordering. Dielectric studies show the presence of a soft mode of relaxation in the vicinity of the B1_OPAF2–B1_TPAF transition, with the relaxation frequency of the mode exhibiting behavior similar to that seen for the smectic A–smectic C* phase transition. Mean field coefficients determining the soft mode behavior as well as the thermal variation of the tilt angle have been determined.