Structures of Three Alkaline-Earth Metal Germanides Refined from Single-Crystal X-ray Diffraction Data

The calcium- and strontium- alumo-germanides SrxCa1-xAl2Ge2 (x asymptotic to 0.4) and SrAl2Ge2 have been synthesized and structurally characterized. Additionally, a binary calcium germanide CaGe has also been identified as a byproduct. All three crystal structures have been established from single-crystal X-ray diffraction methods and refined with high accuracy and precision. The binary CaGe crystallizes with a CrB-type structure in the orthorhombic space group Cmcm (no. 63; Z = 4; Pearson symbol oC8), where the germanium atoms are interconnected into infinite zigzag chains, formally [Ge](2-). The calcium atoms are arranged in monocapped trigonal prisms, centered by Ge atoms. SrxCa1-xAl2Ge2 (x asymptotic to 0.4) and SrAl2Ge2 have been confirmed to crystallize with a CaAl2Si2-type structure in the trigonal space group P3 over bar m1 (no. 164; Z = 1; Pearson symbol hP5), where the germanium and aluminum atoms form puckered double-layers, formally [Al2Ge2](2-). The calcium atoms are located between the layers and reside inside distorted octahedra of Ge atoms. All presented structures have a valence electron count satisfying the octet rules (e.g., Ca2+Ge2- and Ca2+[Al2Ge2](2-)) and can be regarded as Zintl phases.