Heat capacities of the wustites Fe0.9379O and Fe0.9254O at temperatures T from 5 K to 350 K. Thermodynamics of the reactions: xFe(s) + (1/4)Fe3O4(s) = Fe0.7500+xO(s) = Fe1-yO(s) at T [approximate] 850 K, and properties of Fe1-yO(s) to T = 1000 K. Thermodynamics of formation of wustite

Abstract

Thermodynamic properties of wustites 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 wustites were decomposed to iron and iron(II, III) oxide at T [approximate] 800 K and then recombined in the calorimeter. The enthalpy absorption started at T [approximate] 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 [Delta]rHom = (9.04+/-0.25) kJ[middle dot]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 wustite at T 0.90O were estimated and combined with available standard Gibbs free energies of formation for wustite 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.