Nb28Ni33.5Sb12.5, a New Representative of the X-phase

The first ternary compound in the Nb–Ni–Sb system, Nb₂₈Ni_33.5Sb_12.5, has been synthesized and its structure has been determined by single-crystal X-ray diffraction methods. Nb₂₈Ni_33.5(2)Sb_12.5(2) adopts the X-phase structure type (orthorhombic, space group Pnnm, Z=1, a=13.2334(5) Å, b=16.5065(7) Å, c=5.0337(2) Å), which belongs to the set of tetrahedrally close-packed (TCP) structures adopted by many intermetallic compounds. Typical of such TCP structures, the atoms reside in sites of high coordination number, with Ni and Sb in CN12 and Nb in CN14, -15, and -16 sites. The relative importance of various metal–metal bonding interactions is discussed on the basis of extended Hückel band structure calculations. Nb₂₈Ni_33.5Sb_12.5 displays metallic behavior with a room-temperature resistivity of 2.3×10⁻⁴ Ω cm.