Redox and clustering of iron in silicate glasses

Soda–lime–silica glasses (70 mol% SiO2:15 mol% Na2O:15 mol% CaO) containing 0.1–5 mol% Fe2O3 have been investigated using optical absorption spectroscopy, Mössbauer spectroscopy, and wet chemical analysis. Iron redox ratios measured by these different techniques are in agreement. Samples were annealed at 550°C for 1 h in air. As the iron content increases more of the iron ions are in adjacent sites and this manifests itself as changes in the optical and Mössbauer spectra. For example an absorption peak in the visible region which appears with increasing iron content can be assigned to Fe2+–O2−–Fe3+ interactions. Mössbauer parameters indicate that, as Fe2+ and Fe3+ ions cluster, their co-ordination becomes more tetrahedral although the iron concentration has no effect on the Fe2+/ΣFe ratio in the glasses studied. Estimates of the proportion of Fe3+ ions in adjacent sites have been made using Mössbauer data.