SELECTIVE IMAGING OF SUBLATTICES IN COMPLEX STRUCTURES

Several methods used to achieve selective imaging of subsets of atomic columns in complicated structures are reviewed. In many materials selectivity can be achieved as a result of specimen thickness or of defocus or by imaging in superstructure reflections. A recently developed method, especially applicable to intergrowth structures based on two different interpenetrating sublattices, uses selective ''reciprocal lattice sectioning''. In such structures it is possible to excite a single lattice selectively and produce an image by using reflections belonging to that sublattice only. Subsequently, the other sublattice may be excited separately and another image produced. Finally, using reflections which are common to both sublattices it is possible to derive the relationship between the two sublattices and thus describe the modulated intergrowth structure completely. This new technique is the application ''mutatis mutandis'' to electron diffraction of the method sometimes used in X-ray diffraction to determine the structure of such complicated incommensurate compounds. In electron microscopy one has the additional advantage that the atomic-resolution images can be obtained from the two separated substructures and from their geometrical relationship. The method will be illustrated by its applications to a number of intergrowth structures of materials obtained as by-product of the preparation of high T(C) materials: Ca0.85CuO2 and (Sr-Ca)14Cu24O41.