Mossbauer spectra of tin in binary Si-Sn oxide glasses

The Mossbauer spectra of ¹¹⁹Sn in binary glasses of SnO and SiO₂, in varying proportions of between 16.8 and 71.5 mol% SnO, have been obtained at 77 K. From these spectra the tin was found to be predominantly in the 2⁺ state. The isomer shift and the quadrupole splitting were both significantly greater than those in crystalline SnO and were found to decrease with increasing tin content, which also corresponds to decreasing molar volume. A second series of spectra of two of the samples was taken at temperatures between 10 and 300 K. These spectra show a decrease in shift with temperature which was the same for the two samples and which could only partly be accounted for by the second-order Doppler shift. The remainder was ascribed to the temperature dependence of the isomer shift. By using these data together with the measured expansivity and increase in density with tin content, it was possible to correct the isomer shift for the effects of the change in volume, and it was found that the volume-corrected isomer shift increased with increasing tin content. This showed that there was an increase in s-electron density at the tin nuclei, while the accompanying decrease in quadrupole splitting showed a decrease in the p-electron character, as expected for a decrease in covalency. From the decrease in absorption area with temperature the Debye temperature of tin was estimated to be ~190 K, which is slightly less than that of crystalline SnO, and decreased with increasing tin content. All the data suggest that the Sn-O bonds become progressively less covalent as more tin is added to SnO-SiO₂ glasses, and possibly indicate a change from 4-fold to 6-fold co-ordination. A third series of spectra was obtained for the glass containing 40.9 mol% of SnO after a series of heat treatments. The spectra show changes in the oxidation state of the tin which depended upon the different conditions applied.