Study of different modes of dielectric relaxation in the ferro- and antiferroelectric phases of a newly-synthesized highly tilted chiral liquid crystal material 6H6B

Abstract

Frequency- and temperature-dependent dielectric spectroscopy of a newly-synthesised AFLC compound in its planar orientation has been carried out in order to determine the different dielectric parameters and analyse the various modes of dielectric relaxation of the different phases. The compound is (S)-(+)-4′-(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-(6-heptanoyloxyhex-1-oxy)benzoate (acronym 6H6B). It possesses paraelectric SmA* and ferroelectric SmC* phases, in addition to a wide-temperature-range antiferroelectric

phase (∼23°C).

The SmA* phase shows a soft mode of relaxation related to the amplitudons. Its relaxation frequencies are highly temperature-dependent and decrease with decrease in temperature. The dielectric strength is small (∼4–5), but increases with decrease in temperature, and follows the Curie law. The ferroelectric SmC* phase shows a Goldstone mode of relaxation related to the phasons. Its relaxation frequency is almost invariant (∼1–2 kHz) with temperature. The dielectric strength of the Goldstone mode is very large (∼130) and slowly increases with decrease in temperature; it shows a maximum just above the SmC*–SmC_A* transition temperature.

Two modes of dielectric relaxation have been clearly observed in the antiferroelectric

phase. The lower-frequency mode observed in the kHz range has been assigned as the P_L mode, whereas the higher-frequency mode observed at about 100 kHz has been assigned as the P_H mode. Most of the observed relaxations follow a generalised Cole–Cole equation.

Keywords:

• Antiferroelectric liquid crystal
• Dielectric relaxation
• Goldstone mode
• Curie law
• Cole–Cole plot

Acknowledgements

The authors wish to thank the University Grants Commission, New Delhi, and the Department of Science and Technology, New Delhi, for financial assistance. We also thank Mr. A.K. Srivastava for his help during the experiment.