Diffusion of 55Fe in Fe 2O 3 single crystals

Abstract

The diffusion of 55Fe has been measured parallel to the c axis of Fe 2O 3 single crystals at temperatures in the range 708–1303°C and at an oxygen activity of unity. The tracer penetration profiles were determined using sectioning techniques. For temperatures above 900°C the tracer diffusion coefficient is given by D ∗(Fe) = 1.6 × 10 9 exp[−6.0 (eV)/ kT] cm 2 s −1 and below 900°C by 2.8 × 10 −9 exp[−1.8 (eV) kT]. The high-temperature behaviour is probably characteristic of pure Fe 2O 3, whereas diffusion at lower temperatures may be influenced by impurities. The most likely defects responsible for diffusion of Fe are iron interstitials and, for oxygen, oxygen vacancies, and the observed activation energies are discussed in terms of the properties of these defects. The diffusion data and defect models have been used to predict the rate of growth of Fe 2O 3 and indicate that outward Fe diffusion is the dominant transport process. Previously published data for Fe 2O 3 growth in a variety of experimental situations have been corrected to a single rate constant using a model for multilayer growth. The corrected data are all in good agreement but are approximately two orders of magnitude greater than predicted from diffusion data, which suggests that grain boundary diffusion controls the growth of Fe 2O 3 in practice.