| Summary: | The calcium- and strontium- alumo-germanides Sr<i><sub>x</sub></i>Ca<sub>1–<i>x</i></sub>Al<sub>2</sub>Ge<sub>2</sub> (<i>x</i> ≈ 0.4) and SrAl<sub>2</sub>Ge<sub>2</sub> 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 <i>Cmcm</i> (no. 63; <i>Z</i> = 4; Pearson symbol <i>oC</i>8), where the germanium atoms are interconnected into infinite zigzag chains, formally [Ge]<sup>2−</sup>. The calcium atoms are arranged in monocapped trigonal prisms, centered by Ge atoms. Sr<i><sub>x</sub></i>Ca<sub>1−<i>x</i></sub>Al<sub>2</sub>Ge<sub>2</sub> (<i>x</i> ≈ 0.4) and SrAl<sub>2</sub>Ge<sub>2</sub> have been confirmed to crystallize with a CaAl<sub>2</sub>Si<sub>2</sub>-type structure in the trigonal space group <i>P</i><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mover accent="true"><mn>3</mn><mo>¯</mo></mover></semantics></math></inline-formula><i>m</i>1 (no. 164; <i>Z</i> = 1; Pearson symbol <i>hP</i>5), where the germanium and aluminum atoms form puckered double-layers, formally [Al<sub>2</sub>Ge<sub>2</sub>]<sup>2−</sup>. 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., Ca<sup>2+</sup>Ge<sup>2−</sup> and Ca<sup>2+</sup>[Al<sub>2</sub>Ge<sub>2</sub>]<sup>2−</sup>) and can be regarded as Zintl phases.
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