Overlapping SAD patterns

Most computer techniques used to index Selected Area electron Diffraction (SAD) patterns deal with simple patterns in the sense of there being present a single linear array of two reciprocal lattice vectors. If more than one crystal orientation contributes to the pattern, the investigator must judiciously pick and choose pairs of reflections (or groups of reflections if measuring errors are to be minimized) in an effort to sort out which reflections belong to what orientation. Even further complicating the issue is the usual desire to establish the orientational relationship between the different diffracting materials. These problems have been addressed at our laboratories by creating a computer code to isolate and identify overlapping SAD patterns. The basis of our analysis rests upon the fact that each crystal orientation giving rise to an SAD pattern is but a twodimensional sectioning of a much larger three-dimensional reciprocal lattice. At 100 keV operation, the reciprocal lattice vectors associated with each reflection are, to a good approximation, parallel to the vector from the diffraction pattern origin to the reflection in question.