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

Abstract

The calcium- and strontium- alumo-germanides SrxCa1–xAl2Ge2 (x ≈ 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 ≈ 0.4) and SrAl2Ge2 have been confirmed to crystallize with a CaAl2Si2-type structure in the trigonal space group P3¯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.

The Synthesis and Crystal Structure of Six Quaternary Lithium-Alkaline Earth Metal Alumo-Silicides and Alumo-Germanides, A2LiAlTt2 (A = Mg, Ca, Sr, Ba; Tt = Si, Ge)

“…Si-Li/Al and Ge-Li/Al distances are about 10% longer than the sum of the single-bonded Pauling covalent radii (r Li = 1.225 Å; r Al = 1.248 Å; r Si = 1.173 Å; r Ge = 1.242 Å) [26]. These distances match the reported values in a number of alumo-silicides and alumo-germanides, such as Li 8 Al 3 Si 5 [27,28], LiAlSi [28], CaAl 2 Si 2 [29,30], SrAl 2 Si 2 [31,32], BaAl 2 Si 2 [33], LnAl 2 Si 2 (Ln = Y, Sm, Tb, Dy, Yb) [34], Ln 2 Al 3 Si 2 and Ln 2 AlSi 2 (Ln = Y, Tb-Lu) [35], Ba 7.5 Al 13 Si 29 [36], LiAlGe [37], CaAl 2 Ge 2 [38,39], LnAl 2 Ge 2 (Ln = Y, La, Nd, Gd, Tb, Lu) [40], BaAl 2 Ge 2 [41], A 3 Al 2 Ge 2 (A = Ca, Sr, Ba) [42], and LnAl 2 X 2 (Ln = Eu, Yb; X = Si, Ge) [43], among others. Ca atoms have a total coordination number 11, with 6 nearest Li/Al and 5 nearest Tt atoms.…”