In the last ten years, the chemistry and physics of materials with
layered structures became an intensively investigated field in the study
of the solid state. Research into physical properties of these crystals
and especially investigations of their physical anisotropy related to
the structural anisotropy has led to remarkable and perplexing results.
Most of the layered materials exist in several polytypic modifications
and can include stacking faults. The crystal structures are therefore
complex and it became apparent that there was a great need for a review
of the crystallographic data of materials approximating two-dimensional
solids. This second volume in the series 'Physics and Chemistry of
Materials with Layered Structures' has been written by specialists of
different classes of layered materials. Structural data are reviewed and
the most important relations between the structure and the chemical and
physical properties are emphasized. The first three contributions are
devoted to the transition metal dichalcogenides whose physical
properties have been investigated in detail. The crystallographic data
and crystal growth conditions are presented in the first paper. The
second paper constitutes an incisive review of the phase transformations
and charge density waves which have been observed in the metallic
dichalcogenides. In two contributions the layered structures of newer
ternary compounds are de- scribed and the connection between structure
and non-stoichiometry is discussed.