Sn 6 SiO 8 , a Tin(II) Silicate with a Zinc Blende Related Structure and High Thermal Stability

: The crystal structure of a novel cubic tin(II) silicate, Sn 6 SiO 8 (space group F 4 ̅ 3 m , a = 10.40708(2) Å, and Z = 4), synthesized by microwave-assisted hydrothermal synthesis has been solved by Rietveld re ﬁ nement of the powder X-ray di ﬀ raction (PXRD) data. The structure, analogous to zinc blende, comprises a face-centered-cubic array of [Sn 6 O 8 ] 4 − anions, with Si 4+ 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 Sn II oxidation up to ∼ 600 ° C, when the compound begins to thermally

* S Supporting Information ABSTRACT: The crystal structure of a novel cubic tin(II) silicate, Sn 6 SiO 8 (space group F4̅ 3m, a = 10.40708(2) Å, and Z = 4), synthesized by microwaveassisted 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 [Sn 6 O 8 ] 4− anions, with Si 4+ 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 Sn II oxidation up to ∼600°C, when the compound begins to thermally decompose. I n general, tin(II) compounds are relatively rare in the solid state. The only fully characterized tin(II) silicate has the chemical composition Sn 6 SiO 8 and adopts a hexagonal crystal structure with space group P6 3 mc, a = 7.3742(4) Å, and c = 11.9598(10) Å. 1 The phase was not synthesized but rather discovered as one of several corrosion products of pewter that was believed to have spent more than a century submerged in an aqueous environment. The structure comprises [Sn 6  Another tin(II) silicate was observed previously, 2−4 and it had a powder X-ray diffraction (PXRD) pattern that was indexed as cubic and can be found in the ICDD database as pattern PDF 00-020-1295. 2 The phase, first reported in a study on SnO−SiO 2 glass systems, was believed to have the composition SnSiO 3 based on electron microprobe analysis. 3 Further characterization of the compound and elucidation of the crystal structure have not been reported in previous studies where the phase has been synthesized. 2−4 The only reported tin(II) oxyhydroxide, Sn 6 O 4 (OH) 4 , is formed by the alkaline hydrolysis of Sn 2+ salts. The structure comprises discrete Sn 6 O 4 (OH) 4 clusters with hydrogen bonding between the hydroxide and oxide ions on neighboring clusters. The six Sn II atoms of each cluster form an octahedral array with Sn−Sn distances between 3.52 and 3.54 Å; each oxide or hydroxide moiety is coordinated to three Sn atoms that form a face of the Sn 6 octahedron. 5 The material is thermally unstable, and it dehydrates upon heating to form α-SnO at temperatures above 100°C. 6 Clusters containing hexanuclear octahedral arrays of Sn II , such as those in Sn 6 O 4 (OH) 4 , are also observed in organometallic tin(II) compounds with the general formula Sn 6 O 4 (OR) 4 , where R corresponds to methyl, ethyl, or neopentyl moieties. Sn 6 O 4 (OMe) 4 and Sn 6 O 4 (OEt) 4 adopt monoclinic cells, whereas Sn 6 O 4 (ONep) 4 is orthorhombic. 7−9 In the cases of R = Me and Et, the alkoxide and oxide ions bind to the faces of the Sn 6 octahedra in a μ 3 -binding mode, 3,4 in the same manner as hydroxide and oxide in Sn 6 O 4 (OH) 4 . In contrast, neopentoxide ions coordinate in a μ 2 -binding mode to just two Sn atoms of each face owing to steric effects. In these compounds, the clusters are neutral and interact with one another by intermolecular forces. 9 Sn 6 O 4 (OR) 4 clusters bearing organosilicon substituents (R = SiMe 3 ) have been synthesized in solution; however, these clusters are airsensitive, and well-diffracting pure crystals have not been isolated. 10,11 However, diffraction data from a cocrystal, 2Sn 6 O 4 (OSiMe 3 ) 4 ·Sn(OSiMe 3 ) 2 ·4THF, were sufficient to confirm the molecular connectivity but not any details of the molecular geometric parameters. 11 The tin(II) silicate prepared in this work, cubic Sn 6 SiO 8 , was prepared by dissolving NaOH (10 mmol) in 30 mL of deionized water, followed by the addition of fumed SiO 2 (1.67 mmol). Sn(C 2 O 4 ) (10 mmol) was added to the silicate solution with stirring, and the resulting off-white gel was homogenized for 30 min at room temperature before transfer to a Teflon-lined CEM EasyPrep vessel and heating in a CEM Mars 6 microwave oven at 160°C for 30 min (not including 20 min of ramping time) at a microwave power of 600 W. The orange product (0.6 g) was recovered by vacuum filtration, washed with deionized water, and dried for 24 h at 60°C. It has the same PXRD pattern as that of the material previously thought to be SnSiO 3 . 2−4 Because it is the higher-symmetry cubic polymorph of the hexagonal tin(II) silicate discovered in 2006 by Locock et al., we propose that it be referred to as α-Sn 6 SiO 8 to distinguish it from the hexagonal polymorph, which will hereafter be referred to as β-Sn 6 SiO 8 .
The structure of α-Sn 6 SiO 8 has been solved by Rietveld refinement of PXRD data collected on the I11 beamline at the Diamond Light Source. The Rietveld refinement, depicted in Figure 1, was performed for 198 reflections over the 2θ range 6−90°with the GSAS-EXPGUI software 12 using a shifted Chebyschev background function and a pseudo-Voigt profile function with a Finger−Cox−Jephcoat asymmetry correction. The crystallographic data are presented in Table 1 and bond lengths and bond angles in Table 2.
Systematic absences in the indexed PXRD pattern indicate that the space group belongs to the F--extinction group. The candidate space groups were further reduced by the criteria that a Wyckoff position with a multiplicity of 4 must be present, given the stoichiometry and multiplicity of the compound, leaving only two space groups, F4̅ 3m and F23. An initial model for Rietveld refinement was produced by placing atoms on the appropriate Wyckoff positions of the F4̅ 3m space group. Reasonable values for variable fractional coordinates in the Wyckoff positions were deduced by considering the stoichiometry of the compound, geometry, and expected bond lengths and interatomic distances in the related compounds β-Sn 6 SiO 8 and Sn 6 O 4 (OH) 4 .
The structure of α-Sn 6 SiO 8 , depicted in Figure 2, is analogous to zinc blende, comprising a face-centered-cubic array of [Sn 6  The six Sn atoms of each cluster trace a perfect octahedron. The Sn−Sn distance between each Sn atom and the nearest four Sn atoms within the cluster is 3.500(1) Å. A distorted disphenoidal coordination geometry is adopted about each Sn atom, with two shorter equatorial bonds, Sn−O2, to cluster O  The structures of polymorphs α-Sn 6 SiO 8 and β-Sn 6 SiO 8 share many similarities including disphenoidal tin coordination environments, the presence of four-coordinate O ions binding the clusters to Si centers, and the remaining O ions bonding in a μ 3 -binding mode, solely to Sn ions in the cluster. There are also clear structural differences in the polymorphs: only one tin environment and two oxygen environments are present in α-Sn 6 SiO 8 , whereas in β-Sn 6 SiO 8 , there are two tin and four oxygen environments. Consequently, the Sn 6 array in β-Sn 6 SiO 8 does not trace a perfect octahedron unlike the Sn 6 array in α-Sn 6 SiO 8 . As the structure of the cubic polymorph, α-Sn 6 SiO 8 , is analogous to zinc blende, the hexagonal polymorph, β-Sn 6 SiO 8 , is a structural analogue to wurtzite. The similarity to wurtzite was not realized in the initial reports. 1 A diffuse-reflectance (DR) UV−vis spectrum has been recorded for α-Sn 6 SiO 8 (see the Supporting Information, SI). A Tauc plot of the Kubelka−Munk function (see the SI), derived from the DR UV−vis spectrum, 13 indicates that the band gap of α-Sn 6 SiO 8 is 2.42 eV, in agreement with the orange color of the compound.
The Raman spectrum of α-Sn 6 SiO 8 ( Figure 3) contains four peaks at 123, 236, 255, and 348 cm −1 . Similarly, the Raman spectrum of Sn 6 O 4 (OH) 4 contains peaks at 132, 229, and 264 cm −1 , all with comparable relative intensities and within 10 cm −1 of corresponding peaks observed in the α-Sn 6 SiO 8 spectrum. 14 These peaks are likely caused by vibrational modes inherent to the [Sn 6 O 8 ] 4− clusters common to both materials. The peak at 348 cm −1 has no corresponding peak in the Sn 6 O 4 (OH) 4 spectrum and, therefore, likely corresponds to a vibrational mode of the orthosilicate moieties in α-Sn 6 SiO 8 .
Variable-temperature PXRD (Figure 4), recorded on a Bruker D8 diffractometer (reflection mode, Ni-filtered Cu Kα source), reveals that α-Sn 6 SiO 8 is stable up to ca. 600°C in air, at which point it begins to thermally decompose, giving rise to cassiterite, SnO 2 , and an amorphous SiO 2 phase. A PXRD pattern recorded at 600°C matches that recorded at 25°C with no loss of crystallinity. The only reflection at 600°C not attributed to α-Sn 6 SiO 8 is a broad peak of low intensity at 2θ = 26.7°, corresponding to the cassiterite (110) reflection; however, no other cassiterite reflections are present. The intensities of reflections corresponding to α-Sn 6 SiO 8 diminish over the range 630−670°C, and by contrast, the intensities of cassiterite reflections grow markedly over this range. Ultimately, at 670°C, only cassiterite is present. SnO reportedly thermally decomposes in the range 300−500°C, ultimately oxidizing to SnO 2 , initially via mixed-valent tin oxides. 15 The enhanced thermal stability, and resilience to oxidation of Sn 2+ , at relatively high temperatures in α-Sn 6 SiO 8 compared to SnO is likely a result of the enhanced stability afforded by bonding between [Sn 6 O 8 ] clusters and a Si atom in the structure.
In summary, we report the first synthesis of tin(II) silicate, α-Sn 6 SiO 8 , by microwave-assisted hydrothermal synthesis, in addition to a full structural characterization for this previously unsolved structure. α-Sn 6 SiO 8 demonstrates stability at higher temperatures than SnO likely because of the presence of silicate groups within the structure.

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Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.