| Several series of ternary compounds in the (Zr, Hf, Nb)-(Ni, Pd, Pt)-(P, As, Sb) systems, and one family of quaternary antimonides containing three transition metals have been synthesized through reactions of the elements or binary pnictides. Their structures were determined by X-ray diffraction methods and their electronic structures were calculated using the extended Huckel method. This work extends the relatively well studied early transition metal-Ni-pnicogen systems to the much more sparsely investigated Pd and Pt systems. Some of them are the first examples in their ternary or quaternary systems, or adopt new structure types. All compounds are metal-rich phases with extended early-late transition metal bonding interactions and no pnicogen-pnicogen bonding contacts.The structures of Zr3Pd4P3, Hf 3Pd4P3, Nb5Pd4P4, Nb5Pd4As4, Nb9PdAs7, and HfPdSb share the common structural motif of pnicogen-centred trigonal prisms with transition metals as ligands, tp[PnTM 6]. These trigonal prisms are further condensed by sharing edges, triangular faces, or quadrilateral faces. The diversity of structures adopted arises from the differing connectivity of pnicogen-filled trigonal prisms. Zr 3Ni3Sb4, Hf3Ni3Sb4 , and Zr3Pt3Sb4 extend the Y3 Au3Sb4-type structure to compounds containing a non-rare earth component. Nb28Ni33.5Sb12.5 adopts the X-phase structure type, 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 compounds Nb4Pd0.5ZSb2 (Z = Cr, Fe, Co, Ni, Si) represent the first examples of quaternary antimonides containing three transition metals in an ordered arrangement. They are distinguished by the filling of interstitial Z atoms into the centres of Nb8 square antiprismatic clusters that are linked by PdSb4 tetrahedra.All compounds are metallic on the basis of resistivity measurements or predictions from band structure calculations except for Zr3Ni 3Sb4 (or Hf3Ni3Sb4, Zr 3Pt3Sb4), which is predicted to be a small band-gap semiconductor. The Zintl concept and other electron counting rules are not applicable to any of these metal-rich pnictides except for Zr3Ni 3Sb4-type compounds. The chemical bonding, especially the various metal-metal bonding interactions, are discussed on the basis of extended Huckel band structure calculations. |
