Synchrotron x-ray study of surface-aligned smectic A, B, F and crystal G phases in liquid crystals

Abstract We present a surface-oriented synchrotron x-ray diffraction study of the smectic phases in N-(4-n-octyloxybenzylidene)-4′-butylaniline (8O.4) (I-SmA-SmB-SmF-CrG). The x-ray diffraction data from a planar-aligned sample were evaluated to extract orientational order parameters (OOP), layer thickness, and lateral molecular spacing across varying temperatures. The longitudinal and transverse correlation lengths were also determined from the diffraction patterns. Additionally the smectic translational order parameters and the tilt angles of the compound have been measured. The temperature-dependent birefringence (Δn) was measured to assess the orientational order parameter, which was subsequently compared with values derived from x-ray diffraction and mean field calculations. The homeotropically aligned sample yields an in-plane x-ray scattering profile, revealing significant hexatic bond-orientational order in the SmB, SmF and CrG phases. The diffraction patterns were examined to evaluate the bond orientational order (BOO). In the SmB phase, the BOO is approximately 0.85, with an in-plane correlation length of ∼107 Å, suggesting that this phase is a stacked hexatic structure with long-range bond orientational order but short-range positional order. The orthorhombic unit cell parameters for the SmB phase were also determined. Additionally, a new, more ordered smectic phase of this mesogen was identified as the smectic F phase, characterized by a C-centered monoclinic unit cell with axes a > b, where the unique axis b corresponds to the direction of molecular tilt. The technique used in this work will be helpful for future research on higher order smectic phases and other novel phases in liquid crystals.