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 (Mn x Fe 1− x ) 3O 4 at 298 K from the oxides, tetragonal Mn 3O 4 (hausmannite) and cubic Fe 3O 4 (magnetite), is negative from x = 0 to x = 0.67 and becomes slightly positive for 0.67< 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. Δ S 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 . The spinel structure provides octahedral and tetrahedral coordination for cations. In (Mn x Fe 1- x ) 3O 4, strongly negative enthalpies of mixing, measured by oxide melt solution calorimetry, quantify the dominant role of oxidation–reduction reactions (Mn 3++Fe 2+=Mn 2++Fe 3+) in the energetics.