X-ray scattering in smectic liquid crystals: From ideal- to real-structure effects

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Abstract
An analysis is given of the various contributions to the x-ray diffraction in smectic-A liquid crystals, where several effects due to the real structure are superimposed upon the algebraic decay of the correlations due to thermal fluctuations. The finite size of either the sample or intrinsic domains is shown not only to determine the half-width of the Bragg peak but also to contribute to the wings of the diffraction curve, thus overlapping with the algebraic decay. The asymmetry of the peak-intensity distribution observed for nonoriented (powder) samples is also explained. For samples with a small mosaicity, the transition from the limit of powder diffraction to that of a perfectly oriented sample is calculated as a function of the wave-vector magnitude. Finally the possible broadening of the x-ray peak due to dislocations is analyzed. Results of various high-resolution experiments on monomeric, polymeric, and lyotropic smectic liquid crystals are analyzed and the possible sources of observed real-structure effects are discussed.