Theory of reorientational transitions in ferrielectric liquid crystals

A phenomenological model is proposed for the description of the sequences of ferroelectric, antiferroelectric, and ferrielectric smectic phases reported in liquid crystals of the MHPOBC [4-(1-methylheptyloxycarbonyl)phenyl-4’-octyloxybiphenyl-4-carboxylate] type. The model assumes a bilayer smectic ordering and a mechanism consisting of an azimuthal reorientation of the molecules at the transitions. Three helicoïdal ferrielectric structures are found to be possibly stable, in which the helical pitch is shown to vary monotonically as a function of temperature. One of these phases, characterized by an azimuthal mode independent from the z coordinate, provides a suitable interpretation of the presently known results on ferrielectric ordering in liquid crystals. The static and dynamical features of the transitions are worked out and discussed in view of the available data.