Geometrical structure and metal-metal bonding in niobium chalcogenides and chalcogenide halides

An atomic orbital study is presented for the electronic structure of NbS2Cl2, NbSe2Cl2, Nb3Se5Cl7, Nb2Se9 and I0.33NbSe4. Each crystal structure is built up from Nb2X4 clusters (X=S,Se) containing a strong metal-metal bond. Band structures and densities of states are displayed and interpreted in terms of the different geometrical arrangements of the atoms. In particular the crystal stabilities are discussed with reference to the origin of the semiconducting gap for each compound. Metal-metal bonding contributes very strongly to the narrow bands at the edges of the gap.