Crystal structure and electronic spectrum of SnS2

It is shown that layered metal dichalcogenides are quasi-one-dimensional molecular crystals and form a new class of crystal structures — molecular close packed. Since the minimum structural unit in these crystals is a monomolecular layer, using the symbols employed in atomic close packing to describe them gives a mistaken representation of their structure and symmetry. A new system of notation is proposed which provides complete and exact information about the ordering of the atomic layers in different polytype modifications and about their symmetries. It is found that in molecular close packing and, especially, in tin disulfide, there is not one (as in atomic close packing), but two, simplest structures, 1T and 1H, containing one molecule each in a unit cell and, therefore, two series of superlattices constructed on their basis. An energy model is constructed for the natural superlattice in tin disulfide crystals and the electronic spectra of the 2H, 4H, and 9R polytype modifications are calculated in the Kronig-Penney approximation with rectangular potential barriers as superstructure perturbations of the simplest structures. These results make it possible to explain the observed complicated dependence of the band gaps of these crystals on polytype structure.