Square-wave voltammetry and Mِssbauer spectroscopy in tin-doped soda-silicate and soda-lime-silicate glasses

Glasses of the basic compositions xNa2O·(100−x)SiO2 (x=15, 20, 26 and 33 mol%) and xNa2O·10CaO·(90−x)SiO2 (x=10, 16, 20 and 26 mol%) doped with 0.25 mol% SnO2 were studied using square-wave voltammetry in the temperature range from 1000 to 1600 °C. The peak potentials determined by voltammetry were found to decrease linearly with temperature. While the peak potentials of the soda-silicate glasses did not depend on the Na2O-content, those measured in the lime containing glasses decreased with increasing Na2O-concentration. The Mössbauer spectra of tin-doped soda-lime-silicate glasses showed two signals attributed to Sn2+ and Sn4+. The isomer shift for Sn2+ decreases with increasing Na2O-concentration, while for Sn4+ the opposite effect was observed. Both, however, can be explained by an increasing covalent character of the Sn–O bonds with increasing sodium concentration. From isomer shifts and quadrupolar interactions it is concluded that Sn4+ is incorporated in a distorted sixfold coordination with strongly ionic bonds. The redox chemistry of the Sn2+/Sn4+ equilibrium and the incorporation of tin into the silicate network are further discussed.