Tin oxidation state, depth profiles of Sn2+ and Sn4+ and oxygen diffusivity in float glass by Mossbauer spectroscopy

Mossbauer transmission spectra of tin in commercially produced soda-lime-silica float glass have been obtained at low temperatures. It was found that between 20% and 37% of the tin which had diffused into the lower surface of the ribbon during manufacture was in the Sn4+ state. An estimate of the depth profiles for each oxidation state was obtained by measuring the Mossbauer spectra of samples after increasing thicknesses of the tin surface had been removed by polishing. These revealed that about 60% of the total Sn2+ occurs in the first 3.5 mu m, when the total tin penetration depth was 17 mu m. Heat treatments were conducted to investigate the rate of oxidation of Sn2+ to Sn4+ in the glass. The diffusivity of oxygen into the glass was estimated to vary between 1.29 x 10(-18) m(2) s(-1) at 500 degrees C and 1.92 x 10(-16) m(2) s(-1) at 730 degrees C. Measurements of the decrease in recoil-free fraction with temperature enabled values for the Debye temperatures Theta(D) of Sn2+ and Sn4+ to be found to be 185 and 260 K respectively. These results show that the binding of tin at the surface of float glass is weaker than in bulk tin-containing glasses when Theta(D) was 200 K and 319 K.