HEAT-CAPACITY AND THERMODYNAMIC PROPERTIES OF NEARLY STOICHIOMETRIC WUSTITE FROM 13 TO 450 K

The heat capacity of a three-phase sample of Fe0.990+/-0.005O, Fe3O4, and Fe (mole fractions 0.915, 0.078, and 0.007, respectively) has been measured by adiabatic shield calorimetry at temperatures from 13 to 4 50 K. Fe0.99O and magnetite are formed as a metastable intermediate on heating of a quenched nonstoichiometric wustite with composition Fe0. 9374O. The small amount of Fe present stems from the second disproport ionation reaction, in which the stable two-phase mixture of Fe and mag netite is formed from Fe0.99O. The value of Delta S-m (Fe0.99O, 298.15 K), 60.45 J/(K . mol), is derived from the entropy of the three-phase sample and recommended standard entropies of Fe and magnetite. The ch aracter of the magnetic order-disorder transition changes with composi tion and is strongly cooperative in Fe0.99O, with T-N approximate to 1 91 K. A minor, seemingly higher order transition is observed at simila r to 124 K. It is caused by the Verwey transition in the magnetite. Th is magnetite, formed in metastable equilibrium with Fe0.99O, is presum ably more Fe-rich than magnetite in stable equilibrium with Fe.