Crystal Structures of a Cubic Tin(II) Germanate, α-Sn₆GeO₈, and a Tetragonal Tin(II) Silicate, γ-Sn₆SiO₈

A cubic tin(II) germanate, α-Sn₆GeO₈ (space group F4̅3m, a = 10.52521(2) Å, and Z = 4), has been synthesized by both regular hydrothermal and microwave-assisted hydrothermal methods, and the crystal structure of this material has been solved by Rietveld refinement of synchrotron powder X-ray diffraction (PXRD) data. The crystal structure is analogous to α-Sn₆SiO₈ and is therefore related to the zinc blende structure comprising a face-centered cubic array of [Sn₆O₈]^4- anionic clusters with Ge⁴⁺ cations occupying half of the tetrahedral holes. Variable-temperature PXRD has revealed that tin(II) germanate has high thermal stability: remaining stable at 950 K and mostly decomposing over the range 984-1034 K. The tin(II) germanate has been further characterized by X-ray fluorescence (XRF), Raman, and diffuse reflectance (DR) UV-vis spectroscopies. In addition, variable-temperature PXRD studies have revealed the formation of a tetragonal tin(II) silicate polymorph, γ-Sn₆SiO₈ (space group I4̅, a = 7.30414(6) Å, c = 10.53731(6) Å, and Z = 2), at temperatures below 170 K. The crystal structure of γ-Sn₆SiO₈ has been elucidated by Rietveld refinement. While a transition to a tetragonal polymorph is observed upon cooling α-Sn₆SiO₈, no corresponding transition is observed for α-Sn₆GeO₈, which retains its cubic structure over the probed temperature range.