Kinetics of oxidation of low-index surfaces of magnetite

We report on studies of the kinetics of magnetite oxidation, focusing on the early stages of the process. In order to achieve well-defined experimental conditions, we used low-index surfaces of single crystals of magnetite: (111), (110) and (100). For this study we have developed a new technique utilizing the in situ hot stage of a high-resolution x-ray diffractometer. Other techniques were also employed, including Raman spectroscopy and vibrating sample magnetometry. We found that on all three surfaces a layer of haematite, alpha-Fe2O3, was formed as a result of annealing at 300degreesC in air. The layers of oxides formed on all three low-index surfaces of magnetite are non-epitaxial. The layer thickness was calculated as a function of annealing time. The oxidation rate for the (100) surface was significantly greater than for the other two surfaces. Parameters of the oxidation kinetics were obtained through I fitting procedure. The results suggest that the mechanisms of oxidation for these three orientations are different. The oxidation of the magnetite (110) and (111) surfaces is in line with the Wagner model, but this is not the case for the (100) surface. We consider possible mechanisms of oxidation and compare our results with the results of oxidation in powder magnetite.