Goldstone Mode Relaxation in the Ferroelectric Phases of 8OSI

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Abstract
The ferroelectric properties of the Sm C*, Sm I*, Sm J* and Sm K* phases of 4-(2-methylbutyl)-phenyl-4′-(octyloxy)-(l,l′)-biphenyl-carboxylate (80SI) have been studied by dielectric spectroscopy. We observed the bias field dependence of the Goldstone mode and a weak low frequency molecular relaxation. The Sm C* and Sm I* phases showed a composed absorption peak with a critical frequency of ca. 1 kHz. In the highly ordered Sm J* phase there is only a weak absorption peak at about 0.8 kHz. In the Sm K* phase no Goldstone mode has been detected, but only a contribution coming from a molecular relaxation process. The Goldstone mode could be suppressed by an electrical field of about 30 kV/cm. The study of the Goldstone mode was performed in the presence of an aligning magnetic field of 1.2 T parallel to the helix and also without magnetic field. It was found that a magnetic field of 1.2 T is sufficient to reduce the Goldstone mode dielectric increment by about 50%. The fits of the Cole-Cole function to the dielectric spectrum allowed us to calculate the dielectric increments and the critical frequencies of the Goldstone mode and of the molecular relaxation process. The critical frequency of the Goldstone mode increases with the bias field, but it is practically temperature independent within one ferroelectric phase. This increase is discussed in terms of elastic moduli and rotational viscosity. The critical frequency of the molecular process obeys an Arrhenius law with an activation energy of (119 ± 15) kJ/mole in the Sm C* and Sm I* phases.