Niobium and molybdenum compounds with high zinc content:: NbZn3, NbZn16, and MoZn20.44

Well-developed crystals of the title compounds were obtained by slow cooling of binary alloys with high zinc content. Their crystal structures have been determined from four-circle X-ray diffractometer data, The Cu3Au-type structure of NbZn3 was confirmed. NbZn16 has TiZn16-type structure: Cmcm, a = 769.6(1) pm, b = 1143.6(1) pm, c = 1166.3(1) pm, Z = 4, R = 0.021 for 943 structure factors and 50 variable parameters. MoZn20.44 crystallizes with a new structure type and 392 atoms in the cubic face-centered cell: F (4) over bar 3m, a = 1846.4(1) pm, R = 0.059 for 768 F values and 64 variables. The atoms are situated at 17 different crystallographic positions. One molybdenum site has icosahedral zinc coordination. The other molybdenum atoms were found on a position with mixed Mo/Zn occupancy. It is coordinated by 14 zinc atoms in the form of a cube where all faces are capped. The zinc atoms have a great variety of coordination polyhedra with between 10 and 16 neighbors of which at most two are molybdenum atoms. Many of the atomic positions are in common with the atomic positions occupied in the structures of Mg44Rh7, Fe22Zn78, Na6Tl, and Mg6Pd. (C) 1999 Academic Press.