Synthesis and structural characterization of A3In2Ge4 and A5In3Ge6 (A=Ca, Sr, Eu, Yb)—New intermetallic compounds with complex structures, exhibiting Ge–Ge and In–In bonding

Reported are the synthesis and the structural characterization of four new polar intermetallic phases, which exist only with mixed alkaline-earth and rare-earth metal cations in narrow homogeneity ranges. (Sr1–xCax)5In3Ge6 and (Eu1–xYbx)5In3Ge6 (x≈0.7) crystallize in the orthorhombic space group Pnma with two formula units per unit cell (own structure type, Pearson symbol oP56). The lattice parameters are as follows: a=13.109(3)–13.266(3) Å, b=4.4089(9)–4.4703(12) Å, and c=23.316(5)–23.557(6) Å. (Sr1–xCax)3In2Ge4 and (Sr1–xYbx)3In2Ge4 (x≈0.4–0.5) adopt another novel monoclinic structure-type (space group C2/m, Z=4, Pearson symbol mS36) with lattice parameters in the range a=19.978(2)–20.202(2) Å, b=4.5287(5)–4.5664(5) Å, c=10.3295(12)–10.3447(10) Å, and β=98.214(2)–98.470(2)°, depending on the metal cations and their ratio. The polyanionic sub-structures in both cases are based on chains of InGe4 corner-shared tetrahedra. The A5In3Ge6 structure (A=Sr/Ca or Sr/Yb) also features Ge4 tetramers, and isolated In atoms in nearly square-planar environment, while the A3In2Ge4 structure (A=Sr/Ca or Eu/Yb) contains zig-zag chains of In and Ge strings with intricate topology of cis- and trans-bonds. The experimental results have been complemented by tight-binding linear muffin-tin orbital (LMTO) band structure calculations.