Self-diffusion in a-Fe203 natural single crystals.

Abstract

Measurements of 180 self-diffusion in hematite (Fe203) natural single crystals have been carried out as a function of temperature at constant partial pressure ao2 = 6.5"10 -2 in the temperature range 890 to 1227 ~ The ao2 dependence of the oxygen self-diffusion coefficient at fixed temperature T = 1150 ~ has also been deduced in the ao2 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 - 0 26 ( 542 (kJ/mol) ] D o (cm2/s) = 2.7.108 ao2' exp,- ~- J 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 - 0 4 ( 911 (kJ/mol) / D" o (cm2/s) = 3.2.1025 ao2' exp,- RT J Experiments performed introducing simultaneously 180 and 57Fe 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 ~ can be described by DFe (cm2/s) 9.2 101~ -056 (578(kJ/mol) ) = 9 ao~ exp,- 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~ 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 x and charged ones.