Crystal structure of Yb2CuGe6 and Yb3Cu4Ge4 and the valency of ytterbium

Graphical abstract: The crystal structure of intermediate valent Yb2CuGe6 and trivalent Yb3Cu4Ge4 compounds on single crystals obtained from the metal flux technique are supported by electronic structure calculations within the local density approximation. -- Highlights: • The single crystal growth of Yb2CuGe6 and Yb3Cu4Ge4 using metal flux technique. • The crystal structures are supported by electronic structure calculations. • Yb atoms exist as intermediate valent in Yb2CuGe6 and trivalent in Yb3Cu4Ge4. -- Abstract: The Yb2CuGe6 and Yb3Cu4Ge4 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 Gd3Cu4Ge4 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