Self-diffusion in α-Fe2O3 Natural Single Crystals

Measurements of (18)O self-diffusion in hematite (Fe(2)O(3)) natural single crystals have been carried out as a function of temperature at constant partial pressure aO(2) = 6.5.10(-2) in the temperature range 890 to 1227 degrees C. The aO(2) dependence of the oxygen self-diffusion coefficient at fixed temperature T = 1150 degrees C has also been deduced in the aO(2) range 4.5.10(-4) - 6.5.10(-1). The concentration profiles were established by secondary-ion mass spectrometry; several profiles exhibit curvatures or long tails; volume diffusion coefficients were computed from the first part of the profiles using a solution taking into account the evaporation and the exchange at the surface. The results are well described by D(O) (cm(2)/s) = 2.7.10(8) a(O2)(-0.26)exp(- 542 (kJ/mol)/RT) From fitting a grain boundary diffusion solution to the profile tails, the oxygen self-diffusion coefficient in sub-boundaries has been deduced. They are well described by D ''(O) (cm(2)/s) = 3.2.10(25) a(O2)(-0.4)exp(- 911 (kJ/mol)/RT) Experiments performed introducing simultaneously (18)O and (57)Fe provided comparative values of the self-diffusion coefficients in volume: iron is slower than oxygen in this system showing that the concentrations of atomic point defects in the iron sublattice are lower than the concentrations of atomic point defects in the oxygen sublattice. The iron self-diffusion values obtained at T > 940 degrees C can be described by D(Fe) (cm(2)/s) = 9.2.10(10) a(O2)(-0.56)exp(- 578(kJ/mol)/RT) The exponent-1/4 observed for the oxygen activity dependence of the oxygen self-diffusion in the bulk has been interpreted considering that singly charged oxygen vacancies V(O)(center dot) are involved in the oxygen diffusion mechanism. Oxygen activity dependence of iron self-diffusion is not known accurately but the best agreement with the point defect population model is obtained considering that iron self-diffusion occurs both via neutral interstitals Fe(i)(x) and charged ones.