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-ray diffraction. The mixtures of Cr2O3 and Co3O4 powders reacted forming mixed oxides with the spinel structure. In addition to an oxide with the spinel structure an oxide with the rock salt structure was identified in the cobalt-rich mixtures, suggesting that these compositions belong to a two-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 Co1.5Cr1.5O4 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-Co oxide 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 1323 K (in air)