Article ID: ISIJINT-2019-097
Activities of FeO and FeO1.5 in Na2O–SiO2–FeO–Fe2O3 melts have been investigated in terms of the coordination structure of iron ions. The melts were placed in Pt containers at 1573 K and equilibrated at partial pressures of oxygen in the range between 10-9 atm and 10-6 atm, and the activities were derived from Fe concentrations in the Pt containers using the activity coefficient of Fe in Pt–Fe alloys reported as a function of molar fraction of Fe. At the same time, the percentages of Fe2+, Fe3+ in octahedral symmetry (Fe3+(oct)) and Fe3+ in tetrahedral symmetry (Fe3+(tetr)) were also measured by Mössbauer spectroscopy. It has been found that the activity coefficients of FeO (γFeO) are larger than those of FeO1.5 (γFeO1.5), suggesting that FeO is prone to liberate from the silicate network more than FeO1.5. It has also been found that the values of γFeO monotonically increase with increasing Fe2+/Fetotal ratio; in contrast, the values of γ FeO1.5 seem relevant to neither Fe3+(oct)/Fetotal nor Fe3+(tetr)/Fetotal ratio. The activity coefficients have been discussed from the perspective of the coordination structure via the effective ionic radii of Fe2+, Fe3+(oct) and Fe3+(tetr). The magnitude of effective ionic radii is in the hierarchy of Fe2+ > Fe3+(oct) > Fe3+(tetr), and thereby the bond strength between iron ion and oxide ion is in the hierarchy of Fe3+(tetr) > Fe3+(oct) > Fe2+. This suggests that Fe2+ ions are more loosely bound to the silicate skeleton than Fe3+(tetr) and Fe3+(oct) ions, which situation would be reflected in the magnitude of the activity coefficients and their dependencies on Fe2+/Fetotal, Fe3+(oct)/Fetotal and Fe3+(tetr)/Fetotal ratios.