Biphenyl alkyloxy benzoates ferroelectric liquid crystals : structural, thermodynamic and dielectric studies of the octyl and the decyl homologous ; electroclinic effect in the N* phase near a N*-SA-SC* multicritical point

Abstract

A new biphenyl alkyloxy benzoate series is synthesized. Thermodynamic, structural and dielectric measurements have been performed on the two homologous n=8 and n=10. For these two compounds, the sequences N*-SC* or N*-SA-SC* are respectively observed under atmospheric pressure. Pressure-Temperature phase diagrams show that these phase transitions appear near a N*-SA-SC* multicritical point : for n=8, the SA phase is induced under moderate pressure ; for n=10, the SA temperature range is less than 1 °C at atmospheric pressure and slightly increases under pressure. The dielectric charcterization detects a relaxation process in the N* phase near the N*-SC* or the N*-SA phase transitions whose temperature dependence of the characteristic parameters are similar to these of the classical soft mode observed near the SA-SC* phase transition. This relaxation process is attributed to an electroclinic effect in the N* phase near the N*-SC* or the N*-SA phase transitions. This is the first dielectric observation of such an effect, furthermore it is observed it is observed in pure compounds and it is of large amplitude. This is probably connected with hte high polarization, the local smectic order and the proximity of a N*-SA-SC* multicritical point.