Observation of iron diffusion in the near-surface region of magnetite at 470 K

Abstract

Experiments are reported, which allow us to quantify the near-surface cation diffusion in (001) oriented Fe 3 O 4 single crystals at temperatures between 470 and 770 K. Thin homoepitaxial films of magnetite, grown using isotopically labeled 57 Fe, were investigated by neutron reflectivity and time-of-flight secondary ion mass spectrometry. By heating the thin films in high vacuum to different temperatures for a well-defined time and determining the 57 Fe distribution along the surface normal, the diffusion lengths are obtained. For the investigated temperature range, diffusion constants of the order of 10 −20 m 2 /s are deduced. These results are important in view of near-surface mass transport induced by oxygen chemical potential differences occurring when magnetite is exposed to different gas atmospheres or by adsorbates.

Surface structure of magnetite (111) under oxidizing and reducing conditions

“…This may be linked to the growth of new magnetite surface layers upon annealing under oxidizing conditions where cations from the bulk diffuse to the surface leaving vacancies in the bulk of the crystal [37]. Under UHV conditions, Fe cation diffusion was already observed at 150 • C for 57 Fe 3 O 4 films on magnetite (001) [38] and during heating at 450 • C of the clean magnetite (001) surface using SXRD and STM to monitor the phase transition from a SCV reconstructed to a non-reconstructed surface [39].…”

Stability of Iridium Single Atoms on Fe3O4(001) in the mbar Pressure Range

“…With the probing depth being 3× the value of the IMFP, the disappearance of the Ir 4f signal corresponds to a Fe 3 O 4 overlayer with a thickness of at least 5.4 nm. While Ir 1 is stable at this temperature in the two-fold coordinated adsorption site in vacuum, Fe cations from the near-surface region still possess sufficient mobility to diffuse to the surface . Excess Fe cations that have diffused to the surface then react with dissociated O, which results in the growth of additional iron oxide layers …”

“…To identify the reason for this change, we investigated how annealing to 473 K affected the surface while increasing the partial gas pressures. Upon reaching 0.1 mbar O 2 at 473 K, the 51 Excess Fe cations that have diffused to the surface then react with dissociated O, which results in the growth of additional iron oxide layers. 29 Figure 6 follows this progress over the course of several measurement iterations.…”

Oxidation-State Dynamics and Emerging Patterns in Magnetite

“…While this is a common observation for magnetite surfaces in experiments, it is even more prominent in simulations. This is mainly due to the limitations of the system size and the associated problem of nowhere to place the charge-balancing ions, which are most likely to be found at defects or grain boundaries somewhere in the bulk . In addition, artifacts due to nonconverging electrostatics are avoided, as a charge-neutral system is usually required in atomistic simulations with force fields when using periodic boundary conditions along with a long-range solver for electrostatics.…”

“…Magnetite surfaces are inherently nonstoichiometric due to the Fe 2+ /Fe 3+ imbalance with respect to oxygen. ,, Consequently, in common force field simulation approaches, surface modification is associated with a nonstoichiometric composition of the resulting simulation system, which as a consequence is typically not charge neutral, as one is usually unable to add or remove the necessary neutralizing charge anywhere in the bulk due to limited system sizes. However, it should be noted that the Verwey temperature was found to be very sensitive to chemical composition, but the inverse spinel structure of magnetite tolerates small deviations from ideal stoichiometry …”