ATOMIC SITE AND SPECIES DETERMINATION USING THE CHANNELING EFFECT IN ELECTRON DIFFRACTION.

The uses of the channelling (or Borrmann) effect in electron diffraction are reviewed. This effect concerns the variation in the intensity of the emission of secondary radiation with the crystallographic direction of a collimated beam of radiation incident on a crystal. A new technique, ALCHEMI (or Atom Location by CHannelling Enhanced Microanalysis) is then described, based on this effect, which can be used to determine the crystallographic site occupied by small concentrations of impurities in crystals. Results are in agreement with those obtained by other methods. The technique is based on the fast electron channelling effect on characteristic x-ray emission in an electron microscope, and is applied here to a typical spinel structure. The classical problems of ordering in spinels, olivines and feldspars have now been studied by this method.