Ag6GeS4X2 (X:: Cl, Br):: surprisingly no filled Laves phases but the first representatives of a new structure type

The two new deeply coloured, air stable, isotypic compounds Ag6GeS4X2 (X: Cl, Br) were obtained in the course of a series of experiments to prepare new "nonmetallic filled Laves phases (= argyrodites)". Although the chemical compositions of the new compounds are in accordance with the desired rare type of doubly halide (X) substituted argyrodites Ag6GeS4X2, their crystal structures are not of the argyrodite type. Instead they are the first members of the new family of "non-metallic filled NaHg2 phases", i.e. the partial structure of the non-metal atoms S and X (Cl, Br) corresponds to the arrangement of Na and Hg atoms in NaHg2 (= X2S4) with Ge located in tetrahedral holes exclusively and Ag located in tetrahedral as well as octahedral holes. The lattice constants of the new phases are (both space group Pnma, Z = 4): Ag6GeS4Cl2 (a = 643.0(1) pm, b = 761.0(2) pm, c = 2266.0(5) Inn), Ag6GeS4Br2 (a = 654.0(1) pm, b = 772.7(2) pm, c = 2289.9(5) pm). High temperature X-ray powder studies show an irreversible loss of halogen resulting in inhomogeneous decomposition products with argyrodite structure.