The effect of biasing electric field on the soft mode in the vicinity of the ferroelectric phase transition in liquid crystals

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Abstract
The complex permittivity of a room-temperature ferroelectric liquid crystal 4-n-octyloxy benzoic acid 4′-[(2-methylbutyloxy)carbonyl]phenyl ester, has been measured in the vicinity of the phase transition in the frequency range 40 Hz-300 kHz. In the para-electric phase the contribution εs of the soft mode to the permittivity and the soft-mode relaxation frequency f s satisfy the Curie-Weiss law. Under a biasing field E, the helicoidal structure is unwound, and εs and f s can then be measured even in the ferroelectric phase. On the other hand, the phase transition is smeared under the influence of E. This smearing results in deviations from the Curie-Weiss law for both εs and f s in the vicinity of the transition. On increasing E, the maximum of the permittivity, εmax, is lowered and shifted to higher temperatures. Both the shift and ε−1 max are proportional to E 2/3. From experimentally found dependences, some constants in the free energy are determined.