Tribarium tetrahedro-tetragermanide acetylenide, Ba-3[Ge-4][C-2]: Synthesis, structure, and properties

Ba3Ge4C2 is formed at 1530 K from the elements or by reaction of BaC2 with BaGe2 (corundum crucible; steel ampoule). The compound is a semiconductor (grey colour; E-g = 1.1eV), brittle, very sensitive to moisture, and reacts with NH4Cl at about 400 K forming acetylene and germanes up to Ge4Hn. The new Ba3Ge4C2 structure type (space group I4/mcm, No.140; a=8.840(1) Angstrom, c=12.466(1) Angstrom; Z=4, Pearson code tI36), contains two kinds of isolated polyanions, namely tetrahedro-tetragermanide [Ge-4](4-) and acetylenide [C-2](2-) anions. The bond lengths are d(Ge-Ge) = 2.517 Angstrom (4x) and 2.641 Angstrom (2x), and d(C=C)=1.20 Angstrom. The Ba-3[Ge-4][C-2] structure is a hierarchical derivative of the perovskite (CaTiO3) generated by a partial atom/cluster replacement ([Ge-4] for Ca, [C-2] for Ti and Ba for 0). The Raman spectrum shows bands at 168, 199 and 280 cm(-1), and at 1796 cm(-1) characteristic for [Ge-4](4-) and[C-2](2-) Polyanions, respectively.