Concentrations and site partitioning of Fe2+ and Fe3+ ions in a soda-lime-silica glass obtained by optical absorbance spectroscopy

Iron-doped glasses with nominal mol% composition 15Na2O-15CaO-70SiO2, melted under oxidising and reducing conditions, and in most cases refined with sodium sulphate, have been analysed by optical absorbance spectroscopy over a wide (3000-35 000 cm−1) range. The Fe2O3 concentration was varied from 0·05 to 2 mol% and some of the glasses were co-doped with CeO2, reduced with carbon, or oxidised with NaNO3. The developed mathematical fitting analysis revealed that Fe2+ and Fe3+ ions both occupy octahedral and tetrahedral sites. Both types of Fe2+ sites were found to be Jahn-Teller distorted, i.e. elongated, by fitting the transmission absorbance spectra. The spectra of Fe2+ and Fe3+ ions in both types of sites have been fitted as summed peaks of Gaussian shape. All of these spectra, multiplied by the respective ion and site concentrations, are further summed to fit the measured spectra, accurately separated from the fitted background losses. The obtained mathematical description of the Fe spectrum can be used, by modifying only the ion and site concentrations, for analysis of the glasses doped with 0-0·5 mol% Fe2O3, and with only small systematic modifications for concentrations up to 2 mol% Fe2O3. For the first time, the spectra of both Fe2+ and Fe3+ ions in tetrahedral and octahedral sites have been fitted separately. The concentrations of the Fe ions and their sites have been obtained with ±0·001 mol% precision (resolution, repeatability) for the test series of glasses. Accuracy could not be established because there is no other method that could provide better than the ±0·3 wt% received with the best chemical analyses. The redox ratio [Fe2+]/[Fe3+] and the proportions of tetrahedral and octahedral sites were found to vary slightly over the 0-2 mol% Fe2O3 dopant range, and to depend upon the melting environment, the addition of refining, reducing or oxidising agents and the co-dopant CeO2. The proportion [Fe3+octa]/[Fe3+tetra] is more sensitive than the partitioning of the Fe2+ sites to the presence of the refining agent, whilst both proportions are affected by the co-dopant CeO2.