MICRODIFFRACTION AS A TOOL FOR CRYSTAL-STRUCTURE IDENTIFICATION AND DETERMINATION

Microdiffraction patterns obtained with a focused and nearly parallel incoherent electron beam allow one to obtain the crystal system and the Bravais lattice and to reveal the presence of glide planes and screw axes of a specimen. These crystallographic features are easily and reliably obtained, in a systematic way, from a few patterns by means of tables and theoretical patterns established for each crystal system. They lead to an extinction symbol which is in agreement with a small number of possible space groups. Usually, these possible space groups belong to different point groups which may be distinguished by study of the intensity distributions in the diffraction patterns. Consequently the space group or a small sub-set of consistent space groups can be deduced. The technique is therefore very well adapted to phase identification. It can be used as a routine technique and applied to most specimens and especially to small particles, and this constitutes its main advantage. It is particularly valuable with specimens which give poor convergent beam electron diffraction (CBED) patterns. It can also be used in connection with CBED.