Synthesis, structure and chemical bonding of CaCo₂Si₂ and BaCo₂Ge₂ two new compounds with ThCr ₂Si₂ structure type

The intermetallic compounds CaCo₂Si₂ and BaCo ₂Ge₂ were prepared by melting mixtures of the elements in an arc-furnace under argon. Single crystals were obtained by special, heat treatment in welded tantalum ampoules. The compounds were investigated by powder and single-crystal X-ray diffraction, Both title compounds crystallize in the ThCr₂Si₂ structure type with space group IA/mmm: a = 3.9161(3) Å c = 9.9184(7) A, wR2 = 0.032, 220 F² values, 9 variable parameters for CaCo₂Si₂; a = 4.0859(3) Å, c = 11.779(2) Å, wR2 = 0.046, 106 F² values, 9 variable parameters for BaCo₂Ge₂. The de-scribed intermetallic compounds demonstrate the tunable distance between the tetrel elements, which is often observed for compounds with ThCr₂Si₂ structure type. In CaCo₂Si₂ a covalent Si-Si bond is present, whereas no Ge-Ge bond occurs in BaCo₂Ge₂. The CaCo ₂Si₂ and BaCo₂Ge₂ are isoelectronic to BaFe₂As₂, the parent compound of a new "122" family of high temperature superconductors. The crystal chemistry and chemical bonding in title compounds is discussed in terms of LMTO band structure calculations and a topological analysis using the Electron Localization Function (ELF).