Visualizing the iron atom exchange front in the Fe(II)-catalyzed recrystallization of goethite by atom probe tomography

The autocatalytic redox interaction between aqueous Fe(II) and Fe(III)-(oxyhydr)oxide minerals such as goethite and hematite leads to rapid recrystallization marked, in principle, by an atom exchange (AE) front, according to bulk iron isotopic tracer studies. However, direct evidence for this AE fro...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2019-02, Vol.116 (8), p.2866-2874
Main Authors: Taylor, Sandra D., Liu, Jia, Zhang, Xin, Arey, Bruce W., Kovarik, Libor, Schreiber, Daniel K., Perea, Daniel E., Rosso, Kevin M.
Format: Article
Language:English
Subjects:
Crystal defects
Crystallites
Crystals
Electron transfer
Exchanging
Goethite
Hematite
Iron
Iron 57
Minerals
Physical Sciences
PNAS Plus
Recrystallization
Tomography
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Summary:The autocatalytic redox interaction between aqueous Fe(II) and Fe(III)-(oxyhydr)oxide minerals such as goethite and hematite leads to rapid recrystallization marked, in principle, by an atom exchange (AE) front, according to bulk iron isotopic tracer studies. However, direct evidence for this AE front has been elusive given the analytical challenges of mass-resolved imaging at the nanoscale on individual crystallites. We report successful isolation and characterization of the AE front in goethite microrods by 3D atom probe tomography (APT). The microrods were reacted with Fe(II) enriched in tracer 57Fe at conditions consistent with prior bulk studies. APT analyses and 3D reconstructions on cross-sections of the microrods reveal an AE front that is spatially heterogeneous, at times penetrating several nanometers into the lattice, in a manner consistent with defect-accelerated exchange. Evidence for exchange along microstructural domain boundaries was also found, suggesting another important link between exchange extent and initial defect content. The findings provide an unprecedented view into the spatial and temporal characteristics of Fe(II)-catalyzed recrystallization at the atomic scale, and substantiate speculation regarding the role of defects controlling the dynamics of electron transfer and AE interaction at this important redox interface.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1816620116