Broadband dielectric measurements on the (R)-1-methylheptyl-6-(4′-decyloxybenzoyloxy)-2-naphthalene carboxylate antiferroelectric liquid crystal

We have studied by means of polarization, tilt and broadband dielectric spectroscopy the liquid crystal (R)-1-methylheptyl-6-(4′-decyloxybenzoyloxy)-2-naphthalene carboxylate. This compound together with a ferrielectric and two antiferroelectric phases shows a broad range, 6 °C, of Sm-${\mathit{C}}_{\mathrm{\alpha }}^{\mathrm{*}}$ phase. The dielectric study shows the presence of a Goldstone mode in the latter mesophase, which in turn would confirm its tilted and helical nature. Other collective modes, such as the soft mode and a mode related to the antiferroelectric ordering and the helical structure, have also been characterized. With respect to the molecular modes, the rotation around the molecular long axis has been studied from the isotropic to the low temperature antiferroelectric phase. This study shows that the molecules are quite rigid and that the dynamics of the motions around the molecular long axis is not greatly influenced by the transitions among the different smectic mesophases. In particular, the thermal activation energy of the process is the same in the paraelectric Sm-A phase and in the complex Sm-${\mathit{C}}_{\mathrm{\alpha }}^{\mathrm{*}}$ phase. However, the frequency shows a jump at the Sm-${\mathit{C}}_{\gamma }^{\mathrm{*}}$→Sm-${\mathit{C}}_{\mathit{A}}^{\mathrm{*}}$ phase transition, which could be due to an increase of the order of the molecular short axes. © 1996 The American Physical Society.