New ternary germanides: The compounds Ln(4)Zn(5)Ge(6) (Ln : Gd,Tm,Lu)

Three new ternary germanides were prepared by heating mixtures of the eleme nts. Gd4Zn5Ge6 (a = 4.249(3), b = 18.663(17), c = 15.423(6) Angstrom), Tm4Z n5Ge6 (a = 4.190(1), b = 18.410(5), c = 15.105(5) Angstrom), and Lu4Zn5Ge6 (a = 4.179(1), b = 18.368(4), c = 15.050(3) Angstrom) are isotypic and crys tallize in a new structure type (Cmc2(1); Z = 4), composed of edge- and cor ner-sharing ZnGe4 tetrahedra. The rare-earth atoms fill channels of the Zn, Ge network running along the a axis and predominantly have an octahedral co ordination of Ge atoms or a pentagonal prismatic environment: of Zn and Ge atoms. The ZnGe4 tetrahedra are orientated to each other so that two of six Ge atoms form pairs, while the other ones have no homonuclear contacts. Th is is in (Ln(3+))(4)(Zn2+)(5)(Ge3-)(2)(Ge4-)(4). LMTO band structure calcul ations support the interpretation of bondings derived from interatomic dist ances. The metallic conductivity of these compounds expected from the elect ronic band structure was confirmed by measurements of the electrical resist ance of Tm4Zn5Ge6.