X-ray diffraction investigation of phase stability in the Co-Cr-O and the Fe-Co-Cr-O systems in air at 1323 K

Mixtures of Co₃O₄, Cr₂O₃, and Fe₂O₃ were annealed in air for times up to 147 h at 1323 K. Phase analysis was performed using X-raydiffraction.
The mixtures of Cr₂O₃ and Co₃O₄ powders reacted forming mixed oxides with the spinel structure. In addition to an oxide with the spinelstructure an oxide with the rock salt structure was identified in the cobalt-rich mixtures, suggesting that these compositions belong to atwo-phase field in the Cr-Co-O phase diagram. In the Cr-rich mixtures, both non-reacted Cr₂O₃ and CoCr₂O₄ were identified.
The decomposition temperatures of Co₃O₄, Co₂CrO₄, and Co_1.5Cr_1.5O₄ were estimated with thermogravimetry. On the basis of the results obtained a part of the Cr-Co-O phase diagram was constructed.Incorporation of Fe cations in the Co-Cr spinel was observed to expand the spinel lattice. Qualitative phase analyses of the Fe-Cr-Cooxide mixtures indicate that connecting the phase fields in the Fe-Co-O and Cr-Co-O system gives a fair prediction of the phase fields in the Fe-Cr-Co-O system at 1323K (in air).