Measurement of three-dimensional intensity data in electron diffraction by the precession technique

The precession technique, recently devised by Vincent and Midgley at the University of Bristol, has been used to acquire three-dimensional intensity data of sufficient quality for structure determination of the intermetallic phase AlmFe. Dynamical interactions are effectively reduced using the precession method, and the intensity data can be treated within a kinematical approximation. Diffraction patterns from a total of eight projections were digitised by a CCD camera coupled to an image analysis system, and by combining equivalent reflections from different projections, the intensity data was merged into a three-dimensional data set by a least-squares procedure based on experimental intensity ratios between reflections common to each projection. A three-dimensional set of intensity data is needed for calculations of Patterson maps, and eventually Fourier maps, as a guide for a structure model. The quality of the quantitative data and the factors affecting the methods and calculations are discussed. The preliminary conclusion is that the intensities are expected to be useful for calculating Patterson and Fourier maps, but less so for least-squares refinement.