Activity coefficients at low dilution of CrO, NiO and CoO in melts in the system CaO-MgO-Al₂O₃-SiO₂ at 1400°C: Using the thermodynamic behaviour of transition metal oxides in silicate melts to probe their structure

The activity coefficients at low dilution of NiO, CoO and CrO in ∼30 silicate melt compositions in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS) have been determined with a precision of 3% to 5%, one standard deviation, at 1400 °C by equilibration with the pure metal under controlled oxygen fugacity (Ni and Co) or with Cr₂C₃-graphite-CO. The activity coefficients vary by a factor of two over the range of melt compositions studied, but show no simple correlation with melt chemistry or melt structural descriptors. The variation in activity coefficients with melt composition among NiO, CoO and CrO is nevertheless highly correlated, as it also seems to be with the oxides of other divalent cations of similar ionic radius (e.g., Fe²⁺, Mg). In the case of NiO and CoO, the correlation is essentially perfect within the precision of the measurements, that is, the ratio γ_NiO / γ_CoO is the same for all melt compositions, implying that these two cations occupy a similar distribution of coordination environments in all the investigated CMAS melts. The deviations from a one-to-one correlation of the activity coefficient of CrO with those of NiO and CoO may be due to Cr²⁺ cations occurring in a distribution with more distorted coordination environments, due to the strong Jahn-Teller effect in this cation.