Thermochemistry of iron manganese oxide spinels

Oxide melt solution calorimetry has been performed on iron manganese oxide spinels prepared at high temperature. The enthalpy of formation of (MnxFe1-x)(3)O-4 at 298 K from the oxides, tetragonal Mn3O4 (hausmannite) and cubic Fe3O4 (magnetite), is negative from x = 0 to x = 0.67 and becomes slightly positive for 0.67 < x < 1.0. Relative to cubic Mn3O4 (stable at high temperature) and cubic Fe3O4 (magnetite), the enthalpy of formation is negative for all compositions. The enthalpy of formation is most negative near x = 0.2. There is no significant difference in the trend of enthalpy of formation versus composition for cubic (x < 0.6) and tetragonal (x > 0.6) spinels of intermediate compositions. The enthalpies of formation are discussed in terms of three factors: oxidation-reduction relative to the end-members, cation distribution, and tetragonality. A combination of measured enthalpies and Gibbs free energies of formation in the literature provides entropies of mixing. DeltaS(mix) consistent with a cation distribution in which all trivalent manganese is octahedral and all other ions are randomly distributed for x > 0.5, but the entropy of mixing appears to be smaller than these predicted values for x < 0.4. (C) 2004 Elsevier Inc. All rights reserved.