Résumé
Chiral liquid crystal antiferroelectric phase is undistinguishable from the ferroelectric phase by the classical non-resonant X-ray scattering. We report a very different behavior of the 2D high-resolution X-ray diffraction spectra (SAXS) for a phase with antiferroelectric properties. The new antiferroelectric phase is characterized by the appearing of additional peaks in the neighborhood of the smectic Bragg peak. We investigated the dimesogenic chiral liquid crystal siloxane DSI3-MR11 [1]. The phase sequence was determined by optical textural observation. The existence of different smectic phases was confirmed by dielectric, electrooptic and polarization reversal current measurements. We have focused on X-ray diffraction in the vicinity of the transition from SmC* to the antiferroelectric phase. We investigated both powder sample and material aligned under magnetic field. On cooling, the liquid crystal undergoes a rather rapid phase transition from the SmC* phase to a state displaying typical antiferroelectric properties. The transition is then followed by a slow structural evolution. We propose a structure model for the antiferroelectric phase and we develop a simple explanation for the observed evolution. Note that another peculiar property of DSI3-MR11 was previously reported [1, 2]: in the high temperature region it exhibits a de Vries SmA* phase. Resuming both phenomena, we stress that dimesogens with a soft junction can display physical and structural properties quite different with respect to those of simple mesogenic compounds.