Optimized Structure Parameter Determination From CBED

Abstract

The application of convergent beam electron diffraction (CBED) for structure symmetry determination is now well established. Application of quantitative CBED in crystal structure studies is so far much less developed. However, the 2-dimensional dynamical intensity distributions in the disks depend strongly on the structure factor magnitudes and phases for the reflections involved. This is particularly the case in the general non-systematic many-beam case where a quantitative determination of structure factor magnitudes and phases from CBED in general has to be based on full dynamical many-beam calculations. It has been shown however that for carefully chosen experimental conditions it is possible to find line intensities which can be analysed on a kinematic basis. The 1-dimensional intensity variation along a systematic row has also been utilized to determine a structure factor phase with high accuracy. However, an optimal use of the CBED patterns should utilize the general 2-dimensional intensity distribution. The number of unknown parameters in a full many-beam simulation will in principle be large, the computer time large even on supercomputers and an effective algorithm for searching in the many-parameter room is therefore highly needed.