The coupling between polarization and tilt in homologous series of ferroelectric liquidcrystals

Abstract

At temperatures well below the vicinity of the transition to the ferroelectric phase thecoupling between the spontaneous polarization of the SmC* phase with the director tiltangle exhibits nonlinear behaviour. In a generalized Landau model of ferroelectric liquidcrystals this is theoretically described by two terms in the free enthalpy density expansion:(i) the chiral bilinear or piezoelectric coupling term and (ii) the achiral biquadratic couplingterm, which describes the transversal quadrupolar ordering. We have investigated twohomologous series of ferroelectric liquid crystals of varying phase sequence with respect topiezoelectric and quadrupolar coupling. It is shown that the linear coupling increases withincreasing molecular length, while nonlinear contributions decrease. Effects ofvarying phase sequences N*–TGBA*–SmA*–SmC* versus N*–SmC* are discussed.