Fe-oxidation state in alkali-trisilicate glasses - A Raman spectroscopic study

This study focuses on analysis of local structural arrangements of ferric and ferrous iron in binary alkali-silicate networks, and the effect of changing Fe2+/Fe3+ ratio on the glass structure, investigated by Raman spectroscopy. Three alkali trisilicate glasses, Li2Si3O7, Na2Si3O7 and K2Si3O7, were synthesized with 4.4-6.0 wt% iron oxide, to form three series with the same nominal composition but changing Fe2+/Fe3+ ratio. Structural analyses of the high-wavenumber envelope in Raman spectra of Fe-free alkali-trisilicate analogues have been useful in identifying structural species in the Fe-bearing glasses. Peak fitting of the high-wavenumber region between ca. 820 and 1200 cm− 1 indicates that the increase in ferric iron content is associated with a considerable increase in the Q2 species while the abundance of Q4- and Q3-decreases. However, the most prominent change with the increase in the ferric content is the rise of the peak centred at ca. 980 cm− 1 that we identify as related to the vibration of Fe3+-O-Si linkage, regardless of the coordination geometry around ferric cation. We have observed a roughly linear trend in the change of the Fe3+-related Raman peak area with the increase in the ferric iron content for all three alkali trisilicate series, with increasing slope from K to Li. Nonetheless, there are discrete differences in the dependence due to composition of the glass. Based on the results of this study, the obtained linear calibration trend can be used only for an approximate determination and not as a routine quantification of ferric content.