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.

Sn6SiO8, a Tin(II) Silicate with a Zinc Blende Related Structure and High Thermal Stability.

The crystal structure of a novel cubic tin(II) silicate, Sn6SiO8 (space group F4̅3m, a = 10.40708(2) Å, and Z = 4), synthesized by microwave-assisted hydrothermal synthesis has been solved by Rietveld refinement of the powder X-ray diffraction (PXRD) data. The structure, analogous to zinc blende, comprises a face-centered-cubic array of [Sn6O8]4- anions, with Si4+ occupying half of the tetrahedral holes. The tin(II) silicate has been further characterized by variable-temperature PXRD, demonstrating stability of the structure and resistance to SnII oxidation up to ∼600 °C, when the compound begins to thermally decompose.

Mechanistic Insights into Cs-Ion Exchange in the Zeolite Chabazite from In Situ Powder X-Ray Diffraction.

Zeolites contain extraframework cations that are exchangeable under favorable aqueous conditions; this is the fundamental feature for their application in water purification and necessary to produce cation forms for other applications such as catalysis. Optimization of the process is common, but there is little fundamental understanding based on real-time experiments of the mechanism of exchange for most zeolites. The sodium and potassium forms of zeolite chabazite selectively uptake Cs+ by ion exchange, leading to its application in removing radioactive 137Cs+ from industrial nuclear waste streams, as well as from contaminated environments in the aftermath of the Fukushima and Three Mile Island accidents. In this study, in situ synchrotron powder X-ray diffraction patterns have been collected on chabazite as it undergoes Cs-ion exchange. Applying Rietveld refinement to these patterns has revealed the time-resolved structural changes that occur in the zeolite as exchange progresses, charting the changes in the spatial distribution of the extraframework cations and water molecules in the structure during the reaction. Ultimately, a detailed mechanistic understanding of how this dynamic ion-exchange reaction occurs has been obtained.