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Originally published in Science Express on 24 May 2007
Science 22 June 2007:
Vol. 316. no. 5832, pp. 1726 - 1729
DOI: 10.1126/science.1142525
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Reports

The Structure of Ferrihydrite, a Nanocrystalline Material

F. Marc Michel,1,2* Lars Ehm,1,2 Sytle M. Antao,3 Peter L. Lee,3 Peter J. Chupas,3 Gang Liu,1,4 Daniel R. Strongin,1,4 Martin A. A. Schoonen,1,2 Brian L. Phillips,1,2 John B. Parise1,2,5

Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P63mc; a = ~5.95 angstroms, c = ~9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis {delta}-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.

1 Center for Environmental Molecular Science (CEMS), Stony Brook University, Stony Brook, NY 11794, USA.
2 Department of Geosciences, Stony Brook University, Stony Brook, NY 11794, USA.
3 Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA.
4 Department of Chemistry, Temple University, Philadelphia, PA 19122, USA.
5 Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA.

* To whom correspondence should be addressed. E-mail: fmichel{at}ic.sunysb.edu

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