Crystal Structure of the Type I Clathrate Ba8Ni4Ge42
Winnie K. Wong-Ng, Qingzhen Huang, Igor Levin, Joseph C. Woicik, Xun Shi, Jihui Yang, James A. Kaduk
The crystal structure of Type-I clathrate Ba8Ni4Ge42 has been determined using neutron powder diffraction, transmission electron microscopy (TEM, for possible superlattice), extended X-ray absorption fine structure measurements. Ba8Ni4Ge42 is cubic with the space group Pm-3n and lattice parameter a=10.6769(2) Å (Dx=5.988 g.cm-3). The structure combines two different types of polyhedra: the dodecahedtron (Ge20, 10 pentagonal faces) and the tetrakeidecahedron (Ge24, 12 pentagonal and 2 hexagonal faces). The unit cell contains two Ge20 dodecahedra and six Ge24 tetrakaidecahedra . The Ge20 dodecahedra are linked via the interstitial 6c positions. The framework structure is formed by a tetrahedrally bounded network of Ge whereas Ba atoms reside inside the Ge20 and Ge24 cavities at the 2a and 6d crystallographic positions, respectively. Ni atoms occupy exclusively the 6c positions located on the hexagonal faces of the larger tetrakaidecahedra; no Ni was found in the smaller dodecahedra which consist of pentagonal faces. Electron diffraction in TEM revealed no detectable Ge/Ni ordering.