Self-diffusion in α-Fe2O3 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.

Topotaxial reactions during the genesis of oriented rutile/hematite intergrowths from Mwinilunga (Zambia)

“…This is accomplished through an oxidation process, which offers new sites for accommodation of the excess Fe 3+ cations. The empty sites in the oxygen sublattice serve as the pathways for cation migration, as shown by Fe selfdiffusion studies (Amami et al 1999;Sabioni et al 2005;Hallström et al 2011). It has been demonstrated that at higher temperatures, the diffusion of Fe along the c-axis of (10)…”

Iron oxide surfaces

“…O tracer diffusion reveal that oxygen diffusion is faster than that of Fe [169]. In general it seems that both V O and interstitial Fe can form, but Catlow et al [172] proposed that electronic disorder far outweighs ionic diffusion and is responsible for electric conductivity.…”

Oxygen self-diffusion “fast-paths” in titanite single crystals and a general method for deconvolving self-diffusion profiles with “tails”

“…Consequently, even when high-quality crystals free of apparent cracks and inclusions are chosen for lattice diffusion studies, contributions from ''fast-paths'' may be observed in the form of diffusion profiles exhibiting ''tails'' (Reddy and Cooper, 1982;Cawley et al, 1991;Watson and Cherniak, 1997;Moore et al, 1998;Amami et al, 1999). These ''tailed'' diffusion profiles are evidence that there exists another diffusion pathway besides the crystal lattice.…”

“…Because they used a mathematical model for polycrystalline materials with grain boundaries to fit their data, they made the assumption that the fast-paths are like grain boundaries, even though this assumption was not explicitly stated in their papers. Several similar treatments later appeared, for example for oxygen diffusion in corundum , and oxygen diffusion in hematite (Amami et al, 1999). However, the authors did not discuss in detail their assumptions and reasons for selecting these models.…”

“…In such cases, ignoring the small tails will not appreciably affect interpretation of the measurements in terms of lattice diffusion. In some diffusion studies it has been found that for single crystals of certain minerals (e.g., rutile, forsterite, a-Al 2 O 3 , a-Fe 2 O 3 , titanite), almost all the diffusion profiles are ''tailed'', and tail sizes vary widely (Yurimoto et al, 1992;Moore et al, 1998;Amami et al, 1999;Zhang et al, 2006). In order to obtain meaningful results and extract lattice diffusivities from these concentration profiles, the physical nature of these diffusion ''fast-paths'' needs to be understood in order to tailor the mathematical model used to fit the profiles.…”