Experimental demonstration, using polarized Raman and infrared spectroscopy, that both conventional and de Vries smectic-A phases may exist in smectic liquid crystals with a first-order A-C* transition

Abstract

Two models exist for the orientational distribution of the long molecular axes in smectic- A liquid crystals: the conventional unimodal distribution and the "cone-shaped" de Vries distribution. The de Vries hypothesis provides a plausible picture of how, at a molecular level, a first-order Sm-A to Sm- C* transition may occur, especially if there is little or no concomitant shrinkage of the layer spacing. This work investigates two materials with such transitions: C7 and TSiKN65. The azimuthal distribution of in-layer directors is probed using IR and polarized Raman spectroscopy, which allows us to obtain orientational order parameters. In C7, we observe a discontinuous change in the order parameter, the magnitude of which is small compared with the corresponding change in the in-layer director tilt angle Θ. Assuming that the smectic- A liquid crystal is of the de Vries type, we calculate the Θ required to reproduce the apparent order parameter P2 app, obtained from IR, by using the true order parameter P2 , obtained from polarized Raman scattering. The results indicate that, for C7, the tilt angle so calculated is much smaller than that in the Sm- C* angle and hence de Vries behavior may not be the appropriate explanation in this case. Conversely, we find that TSiKN65 shows a different behavior to C7, which can be explained in terms of the de Vries concept. Thus, we conclude that either type of distribution may exist in Sm-A phases which undergo a first-order transition to the Sm- C* phase. We also discuss the changes in the smectic layer spacing and the orientational order parameters across the Sm-A- Sm- C* phase transition, together with changes in birefringence with applied electric field. © 2006 The American Physical Society.