Resonant x-ray diffraction was carried out at the Se K edge in thick free-standing films of a selenophene liquid crystalline material, revealing detail of the structure of the ferro-, ferri-, and antiferroelectric phases. The ferrielectric phase was shown to have a three-layer superlattice. Moreover, the structure of a lower temperature hexatic phase was established. For the antiferroelectric phase, investigations were also carried out in a planar device configuration. The device allowed resonant scattering experiments to be carried out with and without the application of an electric field and resonant data are compared with electro-optic measurements carried out on the same device.
Resonant x-ray scattering has been used to investigate the interlayer ordering of the antiferroelectric and ferrielectric smectic C* subphases in a device geometry. The liquid crystalline materials studied contain a selenium atom and the experiments were carried out at the selenium K edge allowing x-ray transmission through glass. The resonant scattering peaks associated with the antiferroelectric phase were observed in two devices containing different materials. It was observed that the electric-field-induced antiferroelectric to ferroelectric transition coincides with the chevron to bookshelf transition in one of the devices. Observation of the splitting of the antiferroelectric resonant peaks as a function of applied field also confirmed that no helical unwinding occurs at fields lower than the chevron to bookshelf threshold. Resonant features associated with the four-layer ferrielectric liquid crystal phase were observed in a device geometry. Monitoring the electric field dependence of these ferrielectric resonant peaks showed that the chevron to bookshelf transition occurs at a lower applied field than the ferrielectric to ferroelectric switching transition.
The layer structure in the antiferroelectric, ferrielectric, and ferroelectric phases of a liquid crystal device is reported, together with its electric field-induced deformation. The field-free chevron angle is comparable to the steric tilt angle, but differs significantly from the optical tilt angle. A sharp field threshold is observed for the chevron to bookshelf transition in the antiferroelectric phase at 1.3 V/μm, while layer deformations occur at much lower fields (0.3 V/μm) in the other subphases. Models are proposed for the layer deformations.
High-resolution resonant polarized x-ray diffraction experiments near the sulfur K edge have been performed on free-standing liquid crystal films exhibiting the chiral smectic-${\mathrm{C}}_{\mathrm{FI}2}^{*}$ phase. It is widely accepted that this phase has a four-layer repeat unit, but the internal structure of the repeat unit remains controversial. We report different resolved features of the resonant x-ray diffraction peaks associated with the smectic-${\mathrm{C}}_{\mathrm{FI}2}^{*}$ phase that unambiguously demonstrate that the four-layer repeat unit is locally biaxial about the layer normal and that the measured angle, describing the biaxiality, is in good agreement with optical measurements.
The electric-field-induced structural rearrangement of smectic layers in the antiferroelectric and ferroelectric phases of three different materials is reported. The materials all have high optical tilt angles (around 30\ifmmode^\circ\else\textdegree\fi{}), compared with the steric tilt angles deduced from layer spacing measurements (around 18\ifmmode^\circ\else\textdegree\fi{}). The chevron angles observed in devices agree well with values found for the steric tilt angle across the tilted mesophase range. Electric fields were applied to liquid crystal devices while the smectic layer structures, in both the depth and in the plane of the device, were probed using small angle x-ray scattering. Two separate aspects of the influence of the field on the layer structure were studied. First, the organization of the smectic layers in the antiferroelectric phase is described before, during, and after the application of an electric field of sufficient magnitude to induce a chevron to bookshelf transition. Second, the evolution of the field-induced layer structure change has been investigated as the field was incrementally increased in both the antiferroelectric and ferroelectric phases. It was found that the chevron to bookshelf transition has a distinct threshold in the antiferroelectric phase, but shows low or zero threshold behavior in the ferroelectric phase for all the materials studied.
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