COMPOUNDS WITH THE IR3GE7 STRUCTURE TYPE - INTERPENETRATING FRAMEWORKS WITH FLEXIBLE BONDING PROPERTIES

Reexamination of the E-rich (E = Ga, In) area of the binary systems Ni /Ga and Pd/In yielded the stoichiometric compounds Ni3Ga7 and Pd3In7 w ith the Ir3Ge7 structure type. The structure of the compounds was dete rmined by single-crystal X-ray diffraction methods (space group Im (3) over bar m; Ni3Ga7: a = 8.4285(6) Angstrom, Pd3In7: a = 9.4323(2) Ang strom; Z = 4). The existence of the nonstoichiometric phases NiGa4 and PdIn3 reported earlier could not be confirmed and it is assumed that Ni3Ga7 and Pd3In7 represent the most E-rich compounds in their respect ive systems. The remarkable Ir3Ge7 structure consists of two equivalen t interpenetrating frameworks built from cubes and square antiprisms. Its chemical bonding motifs were analysed by ab initio linear muffin-t in orbital (LMTO) and semiempirical tight-binding extended Huckel band -structure calculations. It was found that the valence electron concen tration plays a crucial role in the structural stability of the Ir3Ge7 type and that, in the case of strong dp bonding between transition me tal and E component, semiconducting representatives can occur. We sugg est that the width of the band gaps is tunable by the choice of consti tuent elements in ternary and quarternary compounds with the Ir3Ge7 st ructure.