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Reexamination of the E-rich (E Ga, In) area of the binary systems Ni/Ga and Pd/In yielded the stoichiometric compounds Ni 3 Ga 7 and Pd 3 In 7 with the Ir 3 Ge 7 structure type. The structure of the compounds was determined by single-crystal X-ray diffraction methods (space group Im3 Å m; Ni 3 Ga 7 : a 8.4285(6) , Pd 3 In 7 : a 9.4323(2) ; Z 4). The existence of the nonstoichiometric phases NiGa 4 and PdIn 3 reported earlier could not be confirmed and it is assumed that Ni 3 Ga 7 and Pd 3 In 7 represent the most E-rich compounds in their respective systems. The remarkable Ir 3 Ge 7 structure consists of two equivalent interpenetrating frameworks built from cubes and square antiprisms. Its chemical bonding motifs were analysed by ab initio linear muffin-tin orbital (LMTO) and semiempirical tight-binding extended Hückel band-structure calculations. It was found that the valence electron concentration plays a crucial role in the structural stability of the Ir 3 Ge 7 type and that, in the case of strong dp bonding between transition metal and E component, semiconducting representatives can occur. We suggest that the width of the band gaps is tunable by the choice of constituent elements in ternary and quarternary compounds with the Ir 3 Ge 7 structure.

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