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

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 ε_s 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 ε_s 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 ε_s and f_s in the vicinity of the transition. On increasing E, the maximum of the permittivity, ε_max, 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.