Measurements of 18O self-diffusion in natural haematite monocrystals were carried out, under a constant O partial pressure, at 890 to 1227C. The O partial-pressure dependence of the O diffusivity at 1150C was also deduced. The concentration profiles were determined by means of secondary-ion mass spectrometry. Several of the profiles exhibited curvatures or long tails. The bulk diffusion coefficients were deduced from the first part of the profiles by using a solution which took account of evaporation and exchange at the surface. These results could be described by:

D (cm2/s) = 2.7 x 108aO2-0.26exp[-542(kJ/mol)/RT]

By fitting a grain-boundary diffusion solution to the profile tails, the O self-diffusion coefficients in sub-boundaries were deduced. These results could be described by:

D (cm2/s) = 3.2 x 1025aO2-0.4exp[-911(kJ/mol)/RT]

Experiments which involved the simultaneous introduction of 18O and 57Fe provided comparative values of the bulk self-diffusivities. It was found that Fe diffused slower than O in this system; thus showing that the concentrations of atomic point defects in the Fe sub-lattice were lower than the concentrations of atomic point defects in the O sub-lattice. The Fe self-diffusion values which were obtained above 940C could be described by the exponent of -¼ which was observed for the O activity dependence of O self-diffusion in the bulk. This was explained by supposing that singly-charged O vacancies were involved in the O diffusion mechanism. The O activity-dependence of Fe self-diffusion was not accurately known, but the best agreement with the point defect population model was obtained by assuming that Fe self-diffusion occurred via both neutral and charged interstitials.

Self-Diffusion in α-Fe2O3 Natural Single Crystals. B.Amami, M.Addou, F.Millot, A.Sabioni, C.Monty: Ionics, 1999, 5[5-6], 358-70