Thermodynamics and phase equilibria involving the spinel solid solution FeXMg1−XCr2O4

Activities of FeCr2O4 in the spinel solid solutions FeXMg1−XCr2O4 (0<X<1) in equilibrium with pure iron and Cr2O3 have been measured in the temperature range 1050 to 1350 K by employing a bielectrolyte solid-state galvanic cell of the type Pt, Fe + FeXMg1−XCr2O4 + Cr2O3//(Y2O3) ThO2/(CaO) ZrO2//Fe + FeCr2O4 + Cr2O3, Pt Activities of both the components exhibit small negative deviation from the ideal behavior, characterized by the regular solution parameter Ωs=−2260 (±200) J/mol. The lattice parameter of the spinel solid solutions quenched from 1200 K was found to obey Vegard’s law. The phase relations in the FeO-MgO-Cr2O3 system have been deduced from the results obtained in this study together with other relevant thermodynamic data from the literature. The tie-lines between the solid solutions with rock salt and spinel structures represent the influence of intercrystalline ion exchange. The tie-lines are skewed toward the FeCr2O4 corner, primarily because of the higher stability of FeCr2O4 compared to MgCr2O4, with respect to their component binary oxides. The oxygen partial pressure corresponding to the two three-phase regions, Fe + FeXMg1−XCr2O4 + Cr2O3 and Fe + FeYMg1−YO + FeXMg1−XCr2O4, have been evaluated as a function of composition at 1200 K.