Hematite-ilmenite (Fe₂O₃-FeTiO₃) solid solutions: the effects of cation ordering on the thermodynamics of mixing

Enthalpies of reaction from lead borate drop solution calorimetry at 1057 K show that endothermic mixing dominates hematite-ilmenite solid solutions with compositions from x_ilm = 0 to 0.65, whereas exothermic mixing dominates the compositional region from x_ilm = 0.65 to 1. The measured enthalpy of mixing is interpreted as arising from two contributions: a positive enthalpy of mixing due to repulsive interaction energies and a negative enthalpy of mixing due to attractive interaction energies between layers. Free energies of mixing have been determined independently from the measured tie lines between nonstoichiometric spinel and the sesquioxide phase at 1573 K. Combination of these free energies of mixing and the experimentally determined enthalpies of mixing suggest that the entropy of mixing is far less positive than that predicted by the maximum configurational entropy implied by the measured site occupancies. -from Authors