SrNi₂Ge and SrNi₃Ge₂ - Two related hexagonal germanides

The strontium nickel germanides SrNi₂Ge and SrNi ₃Ge₂ were prepared by melting mixtures of the elements in an arc-furnace under an argon atmosphere. Single crystals were obtained by special heat treatment in welded tantalum ampoules. The compounds were investigated by X-ray diffractions both on powders and single crystals. SrNi₂Ge crystallizes with the hexagonal GdPt₂Sn structure with space group P6₃/mmc, a = 4.128(1), c = 10.479(6) Å, wR2 = 0.105, 92 F² values, 9 variable parameters. In this structure the nickel and germanium atoms build two-dimensional [Ni₂Ge] layers, which are separated by the Sr atoms. The crystal structure of SrNi ₃Ge₂ can be described as a substitution variant of the YPtAs structure with space group P6₃/mmc, a = 4.0511(1), c = 14.2187(6) Å, wR2 = 0.075, 117 F² values, 12 variable parameters. Two adjacent [NiGe] layers in this structure are interconnected by Ni atoms, forming [Ni₃Ge₂] blocks, which are separated by Sr atoms. The structure of SrNi₂Ge and SrNi₃Ge₂ both are related to the AlB₂-type structure. The crystal chemistry and chemical bonding in these structures is discussed in terms of group-subgroup relation, LMTO band structure calculation, and analysis using the Electron Localization Function (ELF).