Effects of the Iron Content and Redox State on the Structure of Sodium Borosilicate Glasses: A Raman, Mossbauer and Boron K-Edge XANES Spectroscopy Study

The structure of iron-bearing sodium borosilicate glasses with up to 10 mol% FeO has been investigated in the range 0.15 < Fe3+/SFe < 0.95. According to Mossbauer spectroscopy, Fe3+ and Fe2+ are mainly in tetrahedral and octahedral coordination, respectively, although other coordination states exist for both cations. From XANES experiments, we conclude that increasing Fe content and varying redox states have only a minor effect on the relative proportions of BO3 and BO4 units. In Raman spectra, a decrease of the proportion of BO4 species present in danburite-like units (Na2O center dot B2O3 center dot 2SiO2) is found upon increasing iron content and oxidizing state. Whereas the insensitivity of the overall boron speciation to iron content and redox state points to weak interactions between boron and iron, the changes affecting BO4 species do indicate a more subtle interplay between Fe3+ and the other tetrahedrally coordinated cations (Si,B) because of the competition between tetrahedral Fe3+ and B3+ for charge compensation by Na+.