A₄Ge₉ (A = K, Rb) as precursors for Hg-substituted clathrate-I synthesis: Crystal structure of A₈Hg₃Ge ₄₃

The Hg-substituted type-I clathrates A₈Hg_3+xGe _43-x (A = K, Rb) were obtained by solid-state reactions from the corresponding Zintl phases A₄Ge₉ and Hg or HgO. The crystal structures of the compounds were determined by single-crystal and powder X-ray diffraction methods. They crystallize in the space group Pm3n, with a = 10.849(1) Å and x = 0.19(5) for K₈Hg_3+xGe _43-x (1) and a = 10.875(1) Å and x = 0.03(7) for Rb ₈Hg_3+xGe_43-x (2). Specific atomic positions (6c site) of the Ge framework are partially substituted by Hg atoms, whereas the alkali-metal atoms reside at the center of the cavities. Both compounds exceed the electron-counting rule for Zintl phases.