CONDENSED TETRAKAIDECAHEDRAL CLUSTERS - THE CRYSTAL-STRUCTURE OF NITA8SE8

The ternary selenides MTa8Se8 (M = Co, Ni) were prepared from compacte d, pre-reacted mixtures of the elemental components. Coarse crystallin e NiTa8Se8 was obtained from reactions performed in sealed molybdenum crucibles at 1620 K for 2 days with the use of iodine as a transport a gent. The selenides crystallize in space group type Pnma with four for mula units per unit cell. The lattice parameters are a = 2241.3(1) pm, b = 344.22(3) pm, c = 1684.1(1) pm and a = 2247.4(2) pm, b = 342.72(3 ) pm, c = 1690.3(l) pm for NiTa8Se8 and CoTa8Se8 respectively. The str ucture of NiTa8Se8 was determined from X-ray intensities of a single c rystal and refined to R(I) = 0.040 for 1411 structure factors and 104 variables. The highly porous intermetallic partial structure contains columns of face-shared, Ni-centred NiTa9 tetrakaidecahedra which are c overed by selenium atoms exhibiting a one-sided coordination with thre e, four or five tantalum atoms as nearest neighbours. NiTa8Se8 is a me tallic conductor.