Quasiperiodicity in Mixed Layer Structures

Three types of mixed layer systems, two of which contain quasiperiodic structures, are studied by means of electron diffraction and electron microscopy. Where possible results are compared with data from X-ray diffraction. As a mixed layer structure we denote every crystal structure consisting of layers or modules with different structure, chemical composition or width. Using a simple model it is shown how the typical features of diffraction patterns due to such a structure can be interpreted (chapter 2). As an example of a "normal" mixed layer system the homologous series As_2Te _3(GeTe)_{rm n } is studied (chapter 3). It is shown that substitution of As by Sb or Bi leads to new homologous series of structures with a mixed layer character. In a mixed layer structure "quasiperiodicity" (i.e. the lack of translation symmetry along at least one direction, giving rise to reflections at non-fractional positions in the diffraction patterns) need not be due to modulation of an underlying basic structure. It can come about in two distinct ways: (i) the sequence of two types of modules with different widths is not periodic but constitutes a quasiperiodic sequence. Examples are analysed in tetradymite based systems doped with cations (chapter four). Approximate sequences of five- and seven -layer modules are derived from electron diffraction patterns by means of a "strip method" (chapters two and four). (ii) the mixed layer structure is an incommensurate planar intergrowth structure or misfit layer structure in which the layers of the component structures do not fit (are "incommensurate") along at least one of the intra-layer directions. A systematic study is made of a series of prototypic examples: the layered ternary chalcogenides "MTS_3" consisting of orthorhombic component structures MS and TS _2 (chapter five).