Abstract
Infrared spectroscopic measurements were carried out on the cobalt-substituted magnetites (Fe 3+) A(Co 2+ x Fe 2+ 1− x Fe 3+) BO 2− 4, pretreated in oxygen, to investigate as a function of temperature the defect phases γ and their transformation to hematite. It has been found that the defect spinels for which x < 0.30 show a partial vacancy ordering on octahedral sites. Referring to the disappearance of the 720-cm −1 absorption band of the defect phases γ or the appearance of the 470-cm −1 absorption band of αFe 2O 3, we show that the transition temperature γ → α increases with cobalt substitution. By comparison with zinc-substituted magnetites, the divalent cation distribution is shown to be important to vacancy ordering and to setting the temperature of hematite precipitation.
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