Secondary reflections in multiple scattering and their interference

A focusing monochromatic X-ray single-crystal technique (Noromosic technique) is applied to investigate shape and interference phenomena of multiply scattered X-ray reflections. It is shown that this technique allows for rapid elimination of spurious 'peaks' caused by coherent or incoherent multiple-scattering processes (Umweganregung). No rotation of the crystal around the reciprocal vector in question is necessary. Furthermore, it is shown that phase determination can be done in principle if two secondary reflected beams are brought to interference within the crystal. This may generally be realized in the n-beam case of diffraction (n ≥ 4) by changing the wavelength or the lattice geometry. The method is applied to a-phenazine, C₁₂H₈N₂, where four- and six-beam cases occur accidentally without a variation of the wavelength. It is shown that phase determination is possible even in such cases where a certain mosaic spread of the crystal is unavoidable.