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Redox equilibria of iron in Ti-bearing calcium silicate quenched glasses
Authors: H. V. Alberto, J. M. Gil, N. Ayres de Campos and B. O. Mysen
Ref.: J. Non-Cryst. Solids 151, 39-50 (1992)
Abstract: Mossbauer spectroscopy has been employed to evaluate relations between redox equilibria of iron, Ti3 content and degree of polymerization of quenched glasses in the system CaO-TiO2-SiO2-Fe-O. The data show that Fe3+/SIGMAFe is positively correlated with Ti/(Ti+Si), with iron content and with NBO/T (non-bridging oxygen per tetrahedrally coordinated cation) of the quenched glasses. In general, isomer shifts of ferric and ferrous iron are consistent with tetrahedrally coordinated Fe3+ (isomer shift near 0.3 mm/s relative to metallic Fe at 298 K) and octahedrally coordinated Fe2+ (isomer shifts between 1 and 1.1 mm/s). Neither isomer shift nor quadrupole splitting is sensitive to the bulk compositional variables (Ti and Fe content) with the exception of the quadrupole splitting of Fe2+. The latter parameter shows positive correlation with the Ti/(Ti+Si) of the quenched glasses, suggesting increasing distortion of the Fe2+-O polyhedra. The ln(Fe3+/Fe2+) is linearly correlated with 1/T (T is absolute temperature) and with ln(f(O2)) (oxygen fugacity) resulting in an enthalpy of reduction of ferric to ferrous iron in the range of 100-200 kJ/mol. This enthalpy increases with increasing Ti and with increasing total iron content. From the log(Fe3+/Fe2+) vs. oxygen fugacity data, the activity coefficient ratio of Fe3+/Fe2+ is negatively correlated with NBO/T and with Ti content. As the oxygen fugacity decreases below that of air (and, therefore, the Fe3+/SIGMAFe also decreases), the isomer shift of ferric iron begins to increase (from approximately 0.3 mm/s to > 0.4 mm/s) so that, when Fe3+/SIGMAFe < 0.3, the isomer shift values are consistent with ferric iron being in octahedral coordination, whereas with Fe3+/SIGMAFe > 0.5, Fe3+ is in tetrahedral coordination. In the Fe3+/SIGMAFe range 0.3-0.5, Fe3+ probably exists both in tetrahedral and octahedral coordination in the quenched glasses. The data are interpreted to suggest that solution of Ti4+ in iron-bearing silicate quenched glasses enhances the stability of Fe3+ relative to Fe2+. This enhancement is the result of complex solution mechanisms involving both ferrous iron titanate complexing and formation of highly polymerized Ti-complexes in the melts.
