Effect of fluorinated achiral chain length on structural, dielectric and electro-optic properties of two terphenyl based antiferroelectric liquid crystals

Abstract

Two antiferroelectric liquid crystals of different fluorinated chain length have been studied. Although both exhibit antiferroelectric (SmCA*), ferroelectric (SmC*) and paraelectric (SmA*) phases, melting point decreases with increasing achiral chain length but clearing point shows opposite trend. Thermal ranges of SmCA* and SmC* phases exhibit odd-even effect like the dipole moments and spontaneous polarization. Layer spacings, average intermolecular distances and correlation lengths across the smectic planes also increase with chain length. Temperature dependence of layer spacings, x-ray tilt, optical tilt and spontaneous polarization suggest tricritical nature of SmC*–SmA* transition. Dielectric increments decrease with increased achiral chain. Both soft mode and Goldstone mode critical frequencies decrease with decreasing temperature in the lower derivative but opposite behaviour is observed in the higher derivative. Both the compounds show Curie–Weiss behaviour in soft mode near SmC*–SmA* transition temperature (Tc). Goldstone mode critical frequency is much higher in the higher derivative. Both in-phase and anti-phase antiferroelectric modes are observed in SmCA* phase in the higher derivative but in the lower derivative only anti-phase antiferroelectric mode is observed. Optical tilts suggest orthoconic nature of the SmCA* phase of both the compounds. Ferroelectric-antiferroelectric transition temperature decreases with increasing ac field in both compounds.