THERMOGRAVIMETRIC STUDY OF THE NON-STOICHIOMETRIC WÜSTITE FeOX. PSEUDO PHASES W i AND W' i . NEW T-P-X EQUILIBRIUM PHASE DIAGRAM

ABSTRACT  A statistical numerical analysis of thermogravimetric measurements (temperature, mass variation, oxygen pressure) leads to a new equilibrium phase diagram T-P-X of the iron monoxide or wüstite FeOx. Three varieties or pseudo phases W1, W2, W3 are revealed above 911 °C, three others W’1, W’2, W’3 below. This diagram displays their subfields, and shows 25 invariant points between stable phases and 8 possible invariant points between metastable phases. A thermodynamic approach is pointed out, which makes possible to characterize the point defects and their clusters as components of the crystal lattice. ADDENDA and comments in 2021. KEY WORDS: thermogravimetry, high temperatures, thermodynamic equilibrium, second-order transitions, triple points, phase diagram