The three-dimensional band structure of polysulphur-nitride

The band structure of polysulphur-nitride has been calculated using the extended Huckel method of Hoffmann (1963) and the crystal structure reported by Boudeulle (1974). For both a single chain of the polymer and for the full three-dimensional band structure, the Fermi level is found to lie at the centre of an antibonding pi band. In the former case, the calculations predict metallic behaviour due to a symmetry-induced degeneracy at the Brillouin zone boundary. This structure would be highly unstable against a Peierls distortion. However, the structure appears to be stabilized by fairly large S-S and N-N overlaps between atoms on neighbouring chains. When interchain overlaps are included, the band structure changes into that of a semi-metal with Fermi surfaces enclosing isolated electron and hole pockets.