Ferroelectric behavior of orthogonal smectic phase made of bent-core molecules

Abstract

Ferroelectric behavior in the recently reported orthogonal ferroelectric SmA d PF phase in an unsymmetric bent-core molecule with a carbosilane terminal group was studied. The ferroelectricity of the SmA d PF phase was unambiguously confirmed by optical second-harmonic generation (SHG) activity in the absence of an electric field, ferroelectric response, and high dielectric strength. The long-range polar order is a consequence of weakened interlayer coupling due to the formation of carbosilane sublayers, which allows for the parallel order of dipole moments of bent-core molecules in the neighboring layers. It develops in the system gradually through the second-order phase transition from the orthogonal SmA d phase. In the SmA d PF phase the strong surface anchoring results in the splay of polarization across the sample thickness. The polar surface anchoring also brings about strongly thickness dependent polar fluctuations, as proved by the dielectric measurements (Goldstone-like mode). The relaxation frequency and dielectric strength vary more than one order of magnitude with cell thickness; in particular the dielectric strength attains more than 2000 in a 25-µm-thick cell and continues to increase for thicker cells. Simple theory developed qualitatively explains the experimental results, supporting the polarization splay model proposed.

Dielectric Response of the Orthogonal Ferroelectric Smectic Phase in Thin Planar Cells in Low DC Bias Electric Field

“…The decrease is bigger in thicker cells. These results are in agreement with the experimental results, which show that the phase mode frequency decreases with the increasing bias electric field [5]. The results obtained by the model presented in this paper agree with the numerically calculated results [6] where the phase mode frequency was obtained by studying the time dependent relaxation of the polar director from a slightly deformed state atẼ B + Ẽ B , where Ẽ B Ẽ B , Downloaded by [McGill University Library] at 07:31 05 November 2014 back to the equilibrium state atẼ B .…”

“…In the confined geometry the dielectric response of this material shows a decrease in the phase mode frequency when the external DC bias field is increased [5]. The unusual dielectric behavior was obtained theoretically for cells with different thicknesses, assuming that the elastic constant of the bend deformation of the short molecular axis is much lower than the splay elastic constant.…”

“…The dielectric properties of SmAP F at low frequencies are dominated by contributions resulting from the fluctuation of the direction of polarization (phase mode) and from the change in the magnitude of polarization (amplitude mode). In [5] it was observed that both the strength and the frequency of the phase mode decrease with increasing bias field. Usually, in ferroelectric bent-core LC, the relaxation frequency increases and the mode strength decreases with increasing bias field [8,9].…”

“…The results are compared with the previously reported numerical calculations [6] and experimental results [5].…”

“…The ferroelectric SmA phase made of bentcore molecules with a carbosilane terminal group is the highest symmetry ferroelectric material found to date. Ferroelectricity was confirmed by the optical second-harmonic generation activity in the absence of an external electric field, the ferroelectric response and high dielectric strength [5]. Properties of the SmAP F phase were studied in planar cells of thicknesses from 1.6 to 25 μm.…”