High-resolution X-ray diffraction on liquid crystals, with a triple-axis spectrometer, was initiated 4-5 years ago, using rotating-anode sources. The natural extension of this work was to use the same kind of spectrometer at the much more powerful source provided by synchrotron radiation. This work was initiated during excursions to the DORIS storage ring in Hamburg in 1979-1980. The triple-axis spectrometer, built at Risø, is now permanently positioned at DORIS in HASYLAB and series of dedicated beam time are used by the solid-state physics group at Risø. The triple-axis X-ray spectrometer work in general and especially at the synchrotron source is a new field and a portion of this thesis is devoted to a description of the techniques it uses.The experiments described here are studies of the nematic to smectic-A phase transition in liquid crystals. The first is a study of the raononolecular liquid crystal 8S5 (C8H17O-φ-COS-φ-C5H11, where φ denotes a benzene ring). The results of this experiment are compared to those on the bimolecular compounds 8CB (C8H17-φ-φ-CN), 80CB (C8H17O-φ-φ-CN) , and CBOOA (C8H17O-φ-NCH-φ-CN). The second experimental study is one of the reentrance phenomenon in the ternary mixture: 5CT.09:7CB.x: 80CB.91-x; where 5CT(C5H11-φ-φ-φ-Cm) and 7CB(C7H15-φ-φ-CN) have only a nematic phase and not the smectic-A phase. The results are interpreted in terms of Landau theory, which also explains a pressure-driven reentrance phenomenon in certain pure bilayer compounds (ex. 80CB). Finally, a frame is given for discussing the nature of the smectic-A phase and an experiment is proposed to explore the nature of the smectic-A phase together with detailed calculations of (001)- and (002)-lineshapes for the smectic-A phase.
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