Dynamical electron diffraction in substitutionally disordered column structures

For column structures, such as fcc-based alloys viewed along the cube direction, the concept of electron channelling through the atom columns is more and more used to interpret the corresponding HREM images. In the case of (partially) disordered columns, the projected potential approach which is used in the channelling description must be questioned since the arrangement of the atoms along the beam direction might affect the exit wave of the electrons. In this paper, we critically inspect this top-bottom effect using multi-slice calculations. A modified channelling theory is introduced which turns out to be very appropriate for the interpretation of these results. For substitutionally disordered column structures, it is also discussed how to link the chemical composition of the material to statistical data of the HREM image. This results in a convenient tool to discern images taken at different thicknesses and focus values.