Multiple Bragg diffraction in quasicrystals: The issue of centrosymmetry in Al-Pd-Mn.

Abstract

When a crystal is rotated around the scattering vector for a Bragg reflection {bold P}, another reflection {bold H} may be simultaneously excited for a particular value {psi} of the azimuthal angle. The plot of the intensity {ital I}{sub {bold P}} vs {psi} (called the {open_quote}{open_quote}azimuthal plot{close_quote}{close_quote}) shows peaks with asymmetric features from which phases of structure factors can be obtained. Multibeam diffraction data have been obtained with a high-quality Al-Pd-Mn quasicrystal using synchrotron x rays. Rocking widths of 36 arcsec have been obtained, which made it possible to obtain data that could be interpreted using dynamical theory without any need of smearing functions to take into account the mosaic spread or other instrumental resolution effects. The asymmetric features and the peak intensity of all azimuthal plots could be fitted with phases consistent with a noncentrosymmetric structure. This conclusion was reinforced by the results of a multibeam experiment with circularly polarized x rays. This is in contrast with several other diffraction experiments based on intensity measurements in the two-beam case. Possible reasons for this discrepancy are discussed. {copyright} {ital 1996 The American Physical Society.}