Field-induced transformations in the biaxial order of non-tilted phases in a bent-core smectic liquid crystal

The structural and electro-optic investigations of an achiral bent-core molecule in SmAPA phase, in which the polar directors in the neighboring layers are arranged antiferroelectrically, show that it undergoes transformation from one biaxial to another biaxial structure via a quasi-stable uniaxial structure on the application of the electric field. The noncontinuous change in biaxiality is explained by an intermediate state in which the secondary directors in the neighboring layers are perpendicular to each other. Copyright c © EPLA, 2010 Ferroelectricity in tilted smectic phases of chiral molecules predicted on symmetry considerations in 1975 was observed [1] and subsequently extensively investigated. In 1992, Brand et al. [2] suggested another model of ferroelectricity for an orthogonal smectic phase (CP ) formed by non-chiral bent-core (or banana, bow, boomerang) molecules. Such a phase possesses an orthorhombic symmetry (C2v), in which one of the three mirror planes is absent and the spontaneous polarization is parallel to the 2-fold axis. The first experimental evidence of ferroelectricity in non-chiral bent-core smectic LCs was given in 1996 by Niori et al. [3] through measurements of the polarization switching current and of the dielectric permittivity. An important feature of the bent-shaped molecules is their tendency to form tilted smectic phases with macroscopic chirality in the opposite directions [4–6]. Since the first works [1] on banana-shaped molecular systems, a wide variety of different tilted phases such as B1-B8, have been discovered and classified [7–10]. Among these B2 is the SmC-like structure which may form four different structures designated as SmCS/APF/A, where subscripts S/A refer to synclinic/anticlinic tilt and F/A refer to ferroelectric/antiferroelectric orderings in the neighboring (a)E-mail: [email protected] layers. In fact the first evidence of ferroelectricity in a non-chiral system [3] was found in the B2 phase. Dipolar order has also been observed in non-tilted antiferroelectric bent-core smectic systems [11,12]. These are also known as orthogonal SmA-like phases. The advantage of orthogonal smectic phases as compared to the tilted phases is that problems associated with SmC -type materials such as a formation of the chevron structures and defects can be avoided. The orthogonal phases of the bent-core materials consist of SmAPA and SmAPR. SmAPA was first discovered by Eremin et al. [11] who also proposed that the polar directors in the neighboring layers were packed antiferroelectrically and ferroelectricity was induced by the application of the electric field above a threshold value. The SmAPR phase with uniaxial structure was discovered by Pociecha et al. [13], where they postulated a random arrangement of the polar directors among the neighboring layers. Shimbo et al. [14] demonstrated that the biaxiality in the SmAPR phase was induced by the electric field and the structure was shown to transform into that of a ferroelectric structure. They also demonstrated this mode for its use in the new LCD technologies of the next generation of displays [15]. It is easier to align materials in SmAPR and SmAPA similar to the biaxial nematics. In the latter phase, the switching is due to the interaction of the minor director with the electric field, E. Since the