NEARLY STOICHIOMETRIC IRON MONOXIDE FORMED AS A METASTABLE INTERMEDIATE IN A 2-STAGE DISPROPORTIONATION OF QUENCHED WUSTITE - THERMODYNAMICAND KINETIC ASPECTS

Quenched metastable wustites are shown to undergo a two-stage dispropo rtionation reaction on heating. A mixture of nearly stoichiometric iro n monoxide and magnetite is formed during the first stage, which takes place at approximate to 470 K. The resulting nearly stoichiometric Fe 1-yO remains metastable up to approximate to 530 K. Above this tempera ture the stable two-phase mixture of iron and magnetite is slowly form ed. The thermodynamic and kinetic aspects of this two-stage disproport ionation reaction have been studied in a step-wise heated adiabatic ca lorimeter. The decomposition behaviour is rationalized in a Gibbs ener gy of formation representation of stable and metastable phases in the iron-oxygen system. The antiferromagnetic to paramagnetic order-disord er transition which takes place in Fe1-yO at approximate to(1)95 K is found to be greatly influenced by the oxygen content; it becomes much more cooperative as the exact 1:1 stoichiometry is approached.