THEORETICAL-STUDY OF THE ELECTRONIC-PROPERTIES OF NB6I9S AND NB6(H)I9S

The structures of the recently synthesized new chain compounds Nb6I9S and Nb6(H)I9S show interesting distortions from what might be viewed as their ideal arrangements. These have been studied in more detail by extended Huckel calculations on the quasi-one-dimensional chain structures. The electronic structures for the observed crystal structures are compared with that of a hypothetical undistorted structure that also contains two Nb6I9S clusters per repeating unit in the infinite chain. Considering 20 electrons/ideal cluster, a local half-filled block is formed by four bands that become 4-fold degenerate at the zone edge. In the distorted structures degeneracies break off to yield a two (empty) over two (filled) band splitting in Nb6(H)I9S (20 electrons per cluster) and a three (empty) over one (filled) band splitting in Nb6I9S (19 electrons per cluster). Thus electronic effects are responsible for the observed distortions.