Structures and properties of vanadium sulphides

The chalcogenides ( sulphides, selenides, tellurides ) of the transition metals have received considerable attention in the last decennia. This is a consequence of the fact that these materials show a great variety of magnetic and electrical properties. The large number of compounds that can occur in a metal-chalcogen system is related to the fact that the transition metals can occur in more than one valency state. Quite a number of transition-metal chalcogenides have a crystal structure which is closely related to a structure type often referred to as the NiAs structure. In this structure the anions are in a hexagonal close packing, with the cations in the octahedral holes of the anion lattice. If we remove the cations in alternate metal planes a structure referred, to as the Cd(OH) 2 structure is obtained. Compounds with an intermediate structure also occur. In these compounds completely filled metal layers and partly filled metal layers are present. In case of ordering the atoms in these partly filled. layers we have superstructures of the NiAs type. The stability of these compounds and, especially the stability of the NiAs structure warrants a discussion because considerations of the Madelung energy favour the NaCl structure instead of the NiAs structure. According to Goodenough the NiAs structure is stabilized by direct metal-metal interaction along the c-axis. For metal-metal distances shorter than a critical distance Rc delocalization of the d-electrons of the metal occurs. Because of the anisotropic behaviour of the d-orbitals a combination of localized and delocalized d-electrons is possible. On the other hand Franzen assumes the interaction between metal and non-metal as the origin of the stability of the NiAs structure. Huisman ascribe the stability of the chalcogenides with the NiAs structure to the fact that the chalcogenions prefer a trigonal-prismatic environment of cations. They calculated a stabilization due to d-covalency for ions of configuration d°, dl and d2 in a trigonal-prismatic environment. The contributions of metal-metal and metal-chalcogen interactions to the stability of NiAs-like compounds can differ considerably for differnt compounds. For the vanadium sulphides, the subject of this thesis, metal-metal interactions must play an important role