Nb28Ni33.5Sb12.5, a new representative of the X-phase

The first ternary compound in the Nb-Ni-Sb system, Nb28Ni33.5Sb12.5, has be en synthesized and its structure has been determined by single-crystal X-ra y diffraction methods. Nb28Ni33.5(2)Sb12.5(2) adopts the X-phase structure type (orthorhombic, space group Pnnm, Z = 1, a = 13.2334(5) Angstrom, b = 1 6.5065(7) Angstrom, c = 5.0337(2) Angstrom), which belongs to the set of te trahedrally close-packed (TCP) structures adopted by many intermetallic com pounds. Typical of such TCP structures, the atoms reside in sites of high c oordination number, with Ni and Sb in CN12 and Nb in CN14, -15, and -16 sit es. The relative importance of various metal-metal bonding interactions is discussed on the basis of extended Huckel band structure calculations. Nb28 Ni33.5Sb12.5 displays metallic behavior with a room-temperature resistivity of 2.3 X 10(-4) Omega cm. (C) 2001 Academic Press.