Thermodynamic properties of wüstites prepared from iron and iron(III) oxide, have been studied by adiabatic calorimetry. Heat-capacity measurements of metastable Fe0.9379O and Fe0.9254O from T = 5 K to 350 K yielded the following integrated values at T = 298.15 K: [[formula]] The wüstites were decomposed to iron and iron(II, III) oxide at T ≈ 800 K and then recombined in the calorimeter. The enthalpy absorption started at T ≈ 850 K. It needed increased temperature and several days for completion: up to T = 932 K and 7.1 d total for Fe0.9254O, up to T = 898.4 K and 2.1 d for Fe0.9379O, and up to T = 948 K and 3.0 d for Fe0.9254O. By averaging the results for the two last determinations, the molar enthalpy of the eutectoid formation reaction:0.1817Fe+(1/4)Fe3O4 = Fe0.9317O, is ΔrHom = (9.04±0.25) kJ·mol-1. The observed eutectoid formation temperature on heating was 854 K on the iron side and 844 K on the magnetite side. In order to delineate the composition range of wüstite at T < 1270 K, thermodynamic functions for FeO and Fe0.90O were estimated and combined with available standard Gibbs free energies of formation for wüstite and the neighboring magnetite phase at T = 1270 K. The resulting eutectoid composition is Fe0.932±0.004O and the calculated eutectoid temperature is (847±7) K. © 1993 Academic Press Ltd.
