Does the valence state of an ion affect its diffusivity? Part I: Oxygen activity dependence of the diffusion of iron in alumina-doped MgO

To investigate whether a change in the valence state of tracer ions affects their diffusivity or not, the iron tracer diffusion in Al2O3-doped MgO, in which 0.5% of the cations were Al3+ ions, has been studied experimentally. Samples were prepared from high purity aluminum and magnesium nitrates using a chemical solution method and from powders of high purity Al2O3 and MgO. Because the concentration of the Al3+ dopant ions present in the samples was much larger than that of all other impurities, the concentration of the majority point defects, cation vacancies, was determined by the Al3+ concentration. Therefore, when changing the oxygen activity, the diffusivity of iron tracer ions can only be altered by changes in their valence state. Measurements of iron tracer diffusion coefficients were performed as a function of the oxygen activity at 1100 and 1200 °C. The experimental results indicate that the mean diffusivity of iron ions in Al2O3-doped MgO increases with increasing oxygen activity at both temperatures, suggesting that Fe3+ ions diffuse in Al2O3-doped MgO faster than Fe2+ ions.

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