An example of the coexistence of octahedral with tetrahedral metal clusters: the ternary phosphide Ni 4Nb 5P 4. Structural and theoretical analyses

Abstract

The ternary compound Ni 4Nb 5P 4 has been synthesized and investigated by means of X-ray powder diffraction. The tetragonal unit cell dimensions are a = 9.9304(4) Å and c = 3.5243(3) Å (space group I4/m, Z=2). The crystal structure has been refined by the Rietveld method (R F = 0.036 and R p = 0.128 for 210 reflexions and 30 refined parameters) using the structure Cu 4Nb 5Si 4 of Ti 5Te 4 type as related model. In me structure, there is no interaction between the metalloid atoms, while Nb-Nb and Ni-Ni bonding leads to the formation of octahedral Nb 6 and tetrahedral Ni 4 clusters. Along the [001]direction of the tetragonal lattice, the Nb 6 clusters condense via opposite apices while the Ni 4 ones share common edges. The structure can be viewed as a fully original structure with infinite linear chains of Nb 6 octahedra and Ni 4 tetrahedra in a phosphorus network. Up to now, such a coexistence of Nb 6 octahedral together with Ni 4 tetrahedral clusters has never been mentioned in solid state chemistry. Tight-binding calculations have been performed in order to analyse the bonding in this ternary compound. In particular, they show that the valence electron count associated with a Nb 5P 4 repeat unit is 45e, i.e the Ni 0 atoms donate zero electron to the Nb 5P 4 substructure. A survey of ternary compounds which also exhibit metal clusters, based on tetrahedral M 4 units (M = Fe, Ni, Cu) is also reported.