Evidence of a polar cybotactic smectic A phase in a new fluorine substituted bent-core compound

Abstract

The mesomorphic properties of an achiral, fluorine substituted bent-core liquid crystal have been investigated using polarising optical microscopy, electro-optics, X-ray diffraction and dielectric spectroscopy. A new phase, designated as CybAP_A, has been found. This phase is a cluster phase consisting of microscopic enlarged antiferroelectric smectic A type clusters. The CybAP_A phase is observed below a SmA phase and exists irrespective of the alignment layer and the cell thickness. The CybAP_A phase behaves like a nematic phase with low viscosity and exhibits thermal fluctuations. But on applying an electric field, it shows smectic-like textures and exhibits tristable antiferroelectric switching. X-Ray diffraction of aligned samples of this phase confirms the non-tilted orientation of the major director and long range 1D positional order of the molecules. The dielectric spectroscopic studies show the appearance of strong dipolar correlations on approaching the CybAP_A phase from the higher temperature SmA phase, confirming the formation of SmAP_A clusters, which is associated with the onset of phase biaxiality. This provides a significant step forward to achieve the illusive orthorhombic biaxial cybotactic nematic phase from bent-core molecules.