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Mixed Cations and Structural Complexity in (Eu1−xCax)4In3Ge4 and (Eu1−xCax)3In2Ge3The First Two Members of the Homologous Series A2[n+m]In2n+mGe2[n+m] (n, m = 1, 2, ...∞; A = Ca, Sr, Ba, Eu, or Yb)

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posted on 15.02.2010, 00:00 by Tae-Soo You, Paul H. Tobash, Svilen Bobev
Reported are the synthesis and the structural characterization of two members of a new homologous series of polar intermetallic compounds, which exist only with mixed alkaline-earth and rare-earth metal cations. Crystals of (Eu1−xCax)4In3Ge4 (0.35(1) ≤ x ≤ 0.70(1)) and (Eu1−xCax)3In2Ge3 (0.78(1) ≤ x ≤ 0.90(1)) have been grown using a molten In metal flux and structurally characterized by single-crystal X-ray diffraction. (Eu1−xCax)4In3Ge4 adopts the monoclinic Mg5Si6-type structure (space group C2/m, Z = 2, Pearson symbol mS22) with lattice parameters a = 16.874(1)−17.024(2) Å, b = 4.496(3)−4.556(1) Å, c = 7.473(4)−7.540(1) Å, and β = 107.306(10)−105.631(3)°. (Eu1−xCax)3In2Ge3 crystallizes with a novel orthorhombic structure (space group Pnma, Z = 4, Pearson symbol oP32) with lattice parameters in the ranges a = 7.382(2)−7.4010(9) Å, b = 4.452(1)−4.4640(6) Å, and c = 23.684(6)−23.734(3) Å, depending on the Eu/Ca ratio. The polyanionic substructures in both cases are related and are based on InGe4 edge-shared tetrahedra, Ge2 dimers, and bridging In atoms in a nearly square-planar environment. The (Eu1−xCax)4In3Ge4 structure can be viewed as a 1:1 intergrowth of Mo2FeB2-like and TiNiSi-like fragments, whereas (Eu1−xCax)3In2Ge3 can be rationalized as a 2:1 intergrowth of the same structural motifs. Both phases exhibit fairly wide homogeneity ranges and exist only with mixed cations. The experimental results have been complemented by linear muffin-tin orbital tight-binding band structure calculations, as well as an analysis of the observed cationic site preferences.