Self-diffusion of O ions in polycrystalline and single-crystal magnesioferrites was measured by using a gas-solid isotopic exchange technique, with 18O as a tracer, at 975 to 1465C. The bulk diffusion coefficients of polycrystalline magnesio-ferrite (MgO/Fe2O3 mole ratio = 0.95) could be expressed by:

1135 to 1465C:     D (cm2/s) =1.51 x 10-1 exp[-78600/RT]

975 to 1135C:     D (cm2/s) =1.2 x 10-7 exp[-38000/RT]

The bulk diffusion coefficients for the polycrystal, in the high temperature range, were very close to those of single-crystal ferrite having the same composition as the polycrystal. The activation energy for grain-boundary diffusion in this polycrystal was

expected to be greater than that for bulk diffusion. A possible explanation of this unusual behaviour was given in terms of an increased enrichment of Fe2+ ions along the grain boundary, with increasing temperature, which also increased the number of O vacancies.

Oxygen Self-Diffusion in Single and Polycrystalline Magnesio-Ferrites. H.Haneda, I.Shindo, H.Yamamura, S.Shirasaki: Journal of Materials Science, 1984, 19[9], 2948-54