The sulphides of vanadium, niobium, and tantalum

The formulae and structures of the sulphides and a comparison with the oxides formed by these metals illustrate the general points noted at the beginning of this chapter. For example, at least nine crystalline Nb-S phases have been characterized, and none has its counterpart among niobium oxides. There are two forms of NbSi x temperature, NiAs superstructure, high temperature, MnP 

Some of these compounds have complex structures which are not easily described, such as the two forms of V3S and Nb21 Sg. The latter is a compound with metallic properties in which there are six kinds of Nb atoms with from 1 to 4 S neighbours. 

Nb atoms are displaced from the centres of their octahedral coordination groups so that Nb-Nb bonds are formed as zigzag chains perpendicular to the paper as indicated in projection by the broken lines in Fig. 17.8(a). The compound is a metallic conductor Nb—Nb, 2 88 A, compare 2 86 A in the metal. 

It is noteworthy that although S and P are similar in size, and Ti2S and Zr2S are isostructural with Ta2 , yet Ta2S has a unique structure with largely Ta-Ta interactions, that is. very low Ta-S coordination numbers compare the 3-5 close S neighbours of a metal atom in Ti2S. 

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