The Current Status of Phase Determination by Means of Multiple Bragg Diffraction

The feasibility of multibeam diffraction for determining phases of structure factors is assessed on the basis of recent contributions and experiments in progress. It is shown that the method works well in situations in which the global interaction between X-ray photons and the crystal is weak, in which case diffraction takes place by single scattering events, and crystal perfection does not play a role in interpreting the experimental results. Two successful examples of phase determinations using the notion of virtual Bragg scattering are presented. One case is of particular interest, because the crystal (V3Si) is mosaic, and the phases were a priori unknown. Some problems and limitations of the method are encountered when trying to extend this technique to organic crystals of relatively large cell size, with spherical or irregular shape. Some data and calculations are presented for benzil, C14H1002, a crystal isomorphous 0108-7673/86/060533-06501.50 with quartz. It is concluded that the resolution presently available from standard laboratory set-ups is not adequate for application of this method to a crystal like benzil, but that use of synchrotron radiation beams will probably remove these obstacles.