Dynamics of the antiferroelectric smectic-Z_A phase in a ferroelectric nematic liquid crystal

A dynamic light scattering study of director fluctuations in the antiferroelectric (AF) phase of the ferroelectric nematic liquid crystal DIO is reported. The AF phase occurs in a temperature range between nonpolar (paraelectric) and ferroelectric nematic states. The behavior of the observed fluctuation modes is consistent with the smectic-Z_A model of the AF phase - namely, a smectic layer structure defined by fluid layers of alternating electric polarization, with the polarization field and nematic director both oriented parallel to the layer planes. This model has been advanced to explain a large body of previous experimental results on DIO. We discuss the wave vector dependence of the modes in terms of a dynamical theory based on the elastic free energy density of a smectic-C phase in the limit of 90° director tilt and an approximation of the viscous stresses by their form for an incompressible uniaxial fluid. We also present the pretransitional temperature dependencies of the splay, twist, and bend elastic constants and the corresponding viscosities in the paraelectric nematic phase.