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 ⟨θmol⟩. In W504 a small increase in ⟨θmol⟩ 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, ⟨θmol⟩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.