Crystal structure of Yb₂CuGe₆ and Yb ₃Cu₄Ge₄ and the valency of ytterbium

The Yb₂CuGe₆ and Yb₃Cu₄Ge ₄ compounds were synthesized from reaction mixtures using indium as flux. Both powder and single crystal X-ray diffraction data were used to refine the crystal structures. Yb₂CuGe₆ crystallizes in the monoclinic space group C2/m in the La₂AlGe₆ type structure. The lattice parameters are a = 8.0011(16) Å, b = 8.1962(16) Å, c = 10.682(2) Å and β = 100.63(3). The crystal structure of Yb₂CuGe₆ can be described as the intergrowth of fragments Ge dimmers and zig-zag Ge chains. Yb₃Cu₄Ge₄ crystallizes in the orthorhombic space group Immm in the Gd₃Cu ₄Ge₄ type structure. The lattice parameters are a = 4.1302(8) Å, b = 6.5985(13) Å and c = 13.691(3) Å. X-ray absorption near edge spectroscopy (XANES) measurements indicate that Yb in Yb₂CuGe₆ and Yb₃Cu₄Ge₄ exists as intermediate and trivalent states, respectively. The structural refinement of Yb₂CuGe₆ is corroborated by total energy calculations within the local density approximation.