The crystal structures of hydrated and partially dehydrated M3HGe7O16 · nH2O, (M = K, Rb, Cs), determined from powder diffraction data using synchrotron radiation

The structures of three hydrated germanates K3HGe7O16 · 3.1 H2O, Rb3HGe7O16 · 3.1 H2O and Cs3HGe7O16 · 2.3 H2O have been determined in space group I23, no. 197, Z = 8, with a = 15.34960(4) Å, 15.45998(4) Å and 15.56660(4) Å, respectively, from Rietveld refinement of powder diffraction patterns collected using synchrotron radiation. The structure of these germanates consists of a three-dimensional framework of tetrahedrally and octahedrally coordinated Ge atoms, with charge compensating cations and water molecules occupying the interconnecting channels. The cations occupy sites near the centres of two types of window approximately 3.3 Å across. One window is nearly circular whilst the other is more elliptical. On partial dehydration, the body-centred superstructure is lost for the Rb and Cs compounds. The structures of Rb3HGe7O16 · 0.69 H2O and Cs3HGe7O16 · 0.70 H2O have been refined in space group P[unk]3m, no. 215, Z = 1, with a = 7.66990(1) Å and 7.73347(3) Å, respectively. In these materials the window apertures are now identical and are circular. For the partially dehydrated K form a 2 × 2 × 2 face-centred cubic structure forms, with a lattice parameter similar to the body-centred hydrated form, a = 15.0844(2) Å.