Evidence for de Vries structure in a smectic- A liquid crystal observed by polarized Raman scattering

The second- and fourth-order apparent orientational order parameters of the core part of the molecule ⟨P₂⟩ₐₚₚ and ⟨P₄⟩ₐₚₚ, have been measured by polarized vibrational Raman spectroscopy for a homogeneously aligned ferroelectric smectic liquid crystal with three dimethyl siloxane groups in the achiral terminal chain, which shows de Vries--type phenomena, i.e., very large electroclinic effect in the smectic-A (Sm-A) phase and a negligible layer contraction at the phase transition between the Sm-A and Sm-C* phases. The apparent orientational order parameters of the rigid core part of the molecule are extremely small both with and without the external electric field in Sm-A. These results provide evidence for the existence of the de Vries Sm-A phase, where the local molecular director is tilted at a large angle.