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

Abstract

Mixtures of Co 3O 4, Cr 2O 3, and Fe 2O 3 were annealed in air for times up to 147 h at 1323 K. Phase analysis was performed using X-ray diffraction. The mixtures of Cr 2O 3 and Co 3O 4 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 Cr 2O 3 and CoCr 2O 4 were identified. The decomposition temperatures of Co 3O 4, Co 2CrO 4, and Co 1.5Cr 1.5O 4 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).