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 1 057 K show that endothermic mixing dominates hematite-ilmenite solid s olutions with compositions from x(ilm) = 0 to 0.65, whereas exothermic mixing dominates the compositional region from x(ilm) = 0.65 to 1. Th e measured enthalpy of mixing is interpreted as arising from two contr ibutions: a positive enthalpy of mixing due to repulsive interaction e nergies (presumably within a hexagonal layer) and a negative enthalpy of mixing due to attractive interaction energies between layers (i.e., the driving force for ordering). Enthalpies of reaction have also bee n measured on compositions from x(ilm) = 0.6 to 0.85 that have differe nt measured cation distributions. Within the resolution of the measure ments (+/- 3 kJ/mol), the enthalpies of isocompositional samples with varying cation distributions are indistinguishable. This observation s upports significant short-range order. As the driving force for orderi ng increases with increasing ilmenite content, progression from short- range order in the more Ti-poor solid solutions to long-range order in the Ti-rich compositions occurs. The progression from short-range to long-range order is expressed by increasingly negative enthalpies of m ixing. Free energies of mixing have been determined independently from the measured tie lines between nonstoichiometric spinel and the sesqu ioxide phase at 1573 K. This requires modeling the activity of Fe8/3O4 in the nonstoichiometric spinel solid solutions coexisting with hemat ite-ilmenite solid solutions. Combination of these free energies of mi xing and the experimentally determined enthalpies of mixing suggest th at the entropy of mixing is far less positive than that predicted by t he maximum configurational entropy implied by the measured site occupa ncies. These results also support significant short-range order.