The method proposed for phase determination is based on the use of overlapping diffraction discs (convergent beam interferometry) and does not assume weak scattering. The method appears to be most useful for the refinement of the inner ″bonding″ structure factors in crystals whose atoms are not spherical. Model calculations for silicon show the sensitivity of dynamical phases to the bonding structure factors at certain thicknesses. At 100 kV accelerating voltage a single overlapping convergent beam pattern allows many phase differences to be determined, however the reciprocity theorem does not refer these to a common incident beam direction, thus restricting the usefulness of the method. The accuracy of phase determination based on the phase grating approximation at 1 MeV using overlapping diffraction discs is shown to lead to errors of approximately 10% for thicknesses of 50 A of low atomic number specimens. Such a method would allow phase determination of all reflections in a zone (referred to a common incident beam direction) within the lattice resolution limit of the instrument and so suggests a technique for the structure determination of large unit cell biological specimens by high voltage convergent beam electron diffraction.
ASJC Scopus subject areas
- Control and Systems Engineering