The peculiar optic, dielectric and X‐ray diffraction properties of a fluorinated de Vries asymmetric diffuse cone‐model ferroelectric liquid crystal

Abstract

A new semi‐fluorinated chiral smectic liquid crystal, W504, is investigated by electro‐optic, dielectric and X‐ray scattering experiments. It exhibits a huge dielectric soft mode response, strong electroclinic effect and a birefringence which increases considerably with the director tilt angle θ; typical characteristics of a SmA*–SmC* transition following the de Vries asymmetric diffuse cone (ADC) model in which the non‐zero director tilt in SmC* arises through an ordering of tilting directions rather than an actual increase in average molecule tilt ⟨θ_mol⟩. In W504 a small increase in ⟨θ_mol⟩ of about 4° is however detected in the SmC* phase. Although the increase in molecule inclination is much less than the increase in director tilt θ, saturating close to 30°, it leads to a shrinkage of the smectic layers by about 1 Å, a result of the large initial molecule tilt in the SmA* phase, ⟨θ_mol⟩_SmA*≈30°. The tilting transition in W504 is thus mainly an ADC model disorder–order transition, but it also has a component of a structural transition. The semi‐fluorinated molecular structure of W504 leads to a very weak electron density modulation along the layer normal, giving a vanishing form factor in bulk samples which exhibit no (0 0 1) X‐ray scattering peak. In thin films the (0 0 1) peak is however observed, indicating that the electron density modulation is enhanced by the breaking of the head–tail symmetry of the liquid crystal phase at the LC–air interface.

Acknowledgments

J.L. gratefully acknowledges financial support from the Alexander von Humboldt Foundation and from the Swedish Research Council.