title: The structure of ferrihydrite, a nanocrystalline material.
abstract: 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 P6(3)mc; a = approximately 5.95 angstroms, c = approximately 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.
authors.affiliation: Center for Environmental Molecular Science (CEMS), Stony Brook University, Stony Brook, NY 11794, USA. fmichel@ic.sunysb.edu
authors.forename: F Marc
authors.initials: FM
authors.lastname: Michel
authors.forename: Lars
authors.initials: L
authors.lastname: Ehm
authors.forename: Sytle M
authors.initials: SM
authors.lastname: Antao
authors.forename: Peter L
authors.initials: PL
authors.lastname: Lee
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Chupas
authors.forename: Gang
authors.initials: G
authors.lastname: Liu
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Strongin
authors.forename: Martin A A
authors.initials: MA
authors.lastname: Schoonen
authors.forename: Brian L
authors.initials: BL
authors.lastname: Phillips
authors.forename: John B
authors.initials: JB
authors.lastname: Parise