MOSSBAUER-SPECTRA OF TIN IN FLOAT GLASS

Tin is not a major constituent of window glass, but is found at high c oncentrations in the lower surface of float glass which has been in co ntact with the molten tin bath. It does not extend far into the surfac e, but causes the physical and chemical behaviour to differ from that of the upper surface. It is important, therefore, to understand the st ructural role of tin in silicate glasses and thus its effect on variou s properties. Mossbauer spectra were taken of three series of glassy m aterials, namely binary glasses (SnO and SiO2) in varying proportions, re-melted float glass containing tin, and float glass from a producti on plant. The binary glasses contained between 20 and 70% tin which wa s found to be mainly Sn2+, with very small amounts of Sn4+ in some of them. The spectra showed a small decrease in isomer shift with increas e in tin content, which is ascribed to the change in molar volume. The re-melted samples were float glass which was mixed with stannous oxal ate in appropriate conditions to try and maintain tin in the 2+ state, and contained up to 15% tin by weight. The spectra show both Sn4+ and Sn2+ with rather more in the 4+ oxidation state. The change in the sp ectra as a function of temperature revealed a large difference in the f-factor (and hence the chemical binding) of the two states. A series of spectra was taken between 17.5 and 900 K for the sample containing 15% tin. From the absorption as a function of temperature the f-factor was determined for both oxidation states, and hence enabled the relat ive amounts of Sn4+ and Sn2+ present in each sample to be estimated. M easurements of the shift as a function of temperature were also made. The float samples were surface material produced by grinding away all but 0.1 mm of the lower surface of industrially produced float glass. The Mossbauer spectra showed them to be predominantly Sn2+, as expecte d from the reducing atmosphere in the float plant. The concentration a nd oxidation state of the tin may be estimated from the value of the f -factors and isomer shifts.