Nb(4)Pd(0.5)ZSb(2) (Z = Cr, Fe, Co, Ni, Si): The first ordered quaternary variants of the W5Si3-type structure

A family of quaternary (or pseudoquaternary) antimonides Nb(4)Pd(0.5)ZSb(2) (Z = Cr, Fe, Co, Ni, Si) containing up to three transition metals in an or dered arrangement has been prepared by reactions of the elements. These ant imonides are isostructural, crystallizing as substitutional variants of the W5Si3-type structure (tetragonal, space group D-4h(18)-I4/mcm, Z = 4) with unit cell parameters a = 10.4407(3) Angstrom and c = 5.0020(2) Angstrom fo r Nb4Pd0.5Cr0.28(3)-Si0.72Sb2, a = 10.4825(6) Angstrom and c = 4.9543(3) An gstrom for Nb4Pd0.5FeSb2, a = 10.4603(5) Angstrom and c = 4.9457(3) Angstro m for Nb4Pd0.5CoSb2, a = 10.4332(7) Angstrom and c = 4.9649(3) Angstrom for Nb4Pd0.5Ni0.78(1)Sb2, and a = 10.3895(10) Angstrom and c = 4.9634(4) Angst rom for Nb4Pd0.5SiSb2. They are distinguished by the filling of interstitia l Z atoms into the centers of Nb-8 square antiprismatic clusters that are l inked by PdSb4 tetrahedra. The Nb8 square antiprisms share opposite square faces to form one-dimensional chains along the c axis so that Z-Z bonding d istances of similar to2.5 A result. Extended Huckel band structure calculat ions were carried out to interpret the homo- and heteroatomic metal-metal i nteractions in the structure. The resistivity of one member, Nb4Pd0.5SiSb2, was measured, indicating metallic behavior.