SYNTHESIS AND CRYSTAL-STRUCTURE DETERMINATION OF THE NEW TERNARY INTERMETALLIC PHASE NA17CU6GA46.5 (NA102CU36GA279)

Na17Cu6Ga46.5 crystallizes in the trigonal system, space group R3m with a = b = 16.183(6), c = 35.190(9) Å; Z = 6. Diffraction data were collected on a Nonius CAD 3 diffractometer within the octants hkl and hkl of the corresponding A-centered monoclinic cell (a = 14.993(6), b = 16.184(7), c = 25.254(8) Å; β = 119.75(5)°). The 5091 reflections were transformed and averaged into 1216 observed (I > 3σ(I)) in the R3m space group. The structure was solved by direct methods and refined by full-matrix least-squares to a final R(F) = 0.056. The structure of Na102Cu36Ga279 contains icosahedra displaying two types of symmetry and a triply fused icosahedron which forms a very unusual polyhedral complex with a copper atom (M28CuM28). Clusters are linked to each other to form an intricated three-dimensional anionic lattice. Sodium cations lie inside 12-vertex (Friauf), 15-vertex, and 16-vertex polyhedral cavities. This stoichiometric phase has been interpreted in terms of electron-deficient bonding within clusters and localized (2c-2e) bonding between clusters. In association with the extended Hückel molecular orbital (EHMO) calculation, the Wade-Mingos electron counting procedure applies successfully. © 1992.