Thermodynamic and dielectric studies on antiferroelectric liquid crystal (S)-4-(1-methylheptyloxycarbonyl)phenyl 4-(6-heptanoyloxyhex-1-oxy)biphenylate

Abstract

Dielectric relaxation phenomena have been investigated for the antiferroelectric liquid-crystal compound (S)-4-(1-methylheptyloxycarbonyl)phenyl-4-(6-heptanoyloxyhex-1-oxy)biphenylate, exhibiting a paraelectric SmA* phase, a ferroelectric SmC* phase, and a wide-temperature-range (∼54–86.0°C) antiferroelectric phase. Dielectric investigations under planar molecular anchoring have been carried out in the frequency range 0.1 Hz to 10 MHz. The paraelectric SmA* phase shows soft mode relaxation related to amplitudons. Its relaxation frequency is highly temperature dependent, and decreases with decrease in temperature. The dielectric strength obeys Curie–Weiss law. The ferroelectric SmC* phase shows a Goldstone mode of relaxation related to the phasons. The relaxation frequency of this mode lies at ∼3.6 kHz, and is almost invariant with temperature. The dielectric strength of the Goldstone mode slowly increases with decrease in temperature. Two characteristic absorption peaks are seen in the phase: the higher-frequency peak corresponds to antiphase fluctuation of the azimuthal angle of directors in the anti-tilted molecular pairs, whereas the lower-frequency peak corresponds to in-phase fluctuation of the azimuthal angle of directors in the anti-tilted molecular pairs.

Keywords:

• Antiferroelectric liquid crystals
• Transition temperatures
• Dielectric relaxations
• Soft and Goldstone modes
• Curie–Weiss law

Acknowledgements

Financial assistance from the Department of Science and Technology, New Delhi, under a ‘Fast Track Project to Young Scientist’ scheme (PS-06/2003) is thankfully acknowledged. Thanks are also due to A. K. Srivastava and M. B. Pandey for some fruitful discussions on the present work.