Thermodynamics and phase equilibria involving the spinel solid solution Fe_X Mg_1−X Cr₂O₄

Abstract

Activities of FeCr₂O₄ in the spinel solid solutions Fe_XMg_1−X Cr₂O₄ (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 + Fe_XMg_1−X Cr₂O₄ + 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 Ω _s =−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 + Fe_X Mg_1−X Cr₂O₄ + Cr₂O₃ and Fe + Fe_Y Mg_1−Y O + Fe_XMg_1−X Cr₂O₄, have been evaluated as a function of composition at 1200 K.