CRYSTALLOGRAPHY IN THE CONVENTIONAL ELECTRON-MICROSCOPE - MOVING BETWEEN DIRECT AND RECIPROCAL SPACE

A survey is given of the application of transmission electron microscopy of thin solid films to the study of crystallographic problems. The main feature of electron microscopy is the possibility to observe direct and reciprocal space of the same small crystal fragment and to relate one to the other. This is especially helpful in the study of imperfect structures. Moreover, the local chemical composition can be determined with a fair precision by X-ray microanalysis. The possibility of image defects in diffraction contrast under two-beam conditions, selecting either the direct or the strongly diffracted beam, is due to the spatial variation of the deviation from Bragg's condition along the defects, as a result of local lattice deformation. Such diffraction contrast images are maps of the intensity distribution in the selected diffraction spot; they cannot provide information on the structure. They have been highly successful in the study of the geometry of defects [Head, Humble, Clarebrough, Morton and Forwood (1973), in Defects in Crystalline Solids. Amsterdam-London-New York: North-Holland Publishing Co.; Humble (1978), in Diffraction and Imaging Techniques in Materials Science, pp. 315-346. Amsterdam-London-New York: North Holland Publishing Co.]. Structural information can be obtained when applying the high-resolution imaging mode, which consists of forming an image by selecting a large number of spots, each spot giving rise to one or several Fourier components contributing to the final image. The images display the configuration of atom columns, in perfect structures as well as round defects. The possibility of imaging atom columns is based on the local nature of electron diffraction. Due to the smallness of the Bragg angles and to multiple diffraction, the electrons are confined to narrow columns on passing through the foil. The atomic columns constitute cylindrically symmetric potential wells which cause channelling of the electrons. By choosing an appropriate set of reflections one can selectively image the structure associated with each sublattice in crystals built on two distinct sublattices, differing sufficiently in unit-cell parameters. In ordered binary alloys selective imaging in the superstructure reflections reveals the sublattice of the minority atom columns. Examples of applications of these different imaging modes are discussed. Electron microscopy has been especially useful in the study of polytypes, intergrowth structures, mixed-layer compounds and long-period structures in general. Several examples of such studies are discussed in some detail, especially in the case of high T-c superconductors. Electron microscopic methods have also been essential in the study of crystals of which only small fragments are available, such as for instance pure fullerenes during the initial stage of this research area. Although convergent-beam electron diffraction is quite useful for a determination of the space group of small crystallites, we have not reviewed this method due to space limitations.