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.

A new dedicated high-resolution high-throughput powder diffraction beamline has been built, fully commissioned, and opened to general users at the Advanced Photon Source. The optical design and commissioning results are presented. Beamline performance was examined using a mixture of the NIST Si and Al(2)O(3) standard reference materials, as well as the LaB6 line-shape standard. Instrumental resolution as high as 1.7 x 10(-4) (DeltaQQ) was observed.

A dedicated high-resolution high-throughput X-ray powder diffraction beamline has been constructed at the Advanced Photon Source (APS). In order to achieve the goals of both high resolution and high throughput in a powder instrument, a multi-analyzer detector system is required. The design and performance of the 12-analyzer detector system installed on the powder diffractometer at the 11-BM beamline of APS are presented.

Iron(III) hydroxides are abundant in near-surface natural environments and play an important role in geochemical processes and the fate of contaminants. The issue of the structure of the common nanophase material ferrihydrite (ferric hydroxide) is controversial and has been debated in the literature for many years without definitive resolution. At least two types of ferrihydrite, the 2-line and 6-line forms, are conventionally recognized. It has been suggested that these forms possess different structures built up by different mixtures of distinct nanophase components. However, traditional crystallographic methods provide depictions of structure that are most sensitive either to short-range order (X-ray absorption) or long-range periodicity (X-ray diffraction or electron diffraction). We used high-energy X-ray total scattering for pair distribution function analysis to observe both the short- and intermediate-range ordering (exceeding ∼15 Å) of synthetic ferrihydrite with three distinct average domain sizes of 2, 3, and 6 nm. We show that there are no significant differences in the underlying structures of these materials and that the differences in the diffraction patterns can be entirely interpreted by variations in the average size of the coherent scattering domains. The average crystallite sizes inferred from the PDF analysis are in good agreement with direct observation by high-resolution transmission electron microscopy.

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The crystal structures of four isostructural orthorhombic carbonates, CaCO 3 (aragonite), SrCO 3 (strontianite), PbCO 3 (cerussite), and BaCO 3 (witherite), were obtained by Rietveld refinements using data acquired by synchrotron high-resolution powder X-ray diffraction (HRPXRD). For BaCO 3 , powder neutron-diffraction data were obtained and refined by the Rietveld method. For aragonite, we also carried out a refinement of the structure by single-crystal X-ray diffraction. These carbonates belong to the space group Pmcn, with Z = 4. The CO 3 group is slightly non-planar, and the two independent C-O distances are slightly different. The CO 3 group becomes more symmetrical and less aplanar from CaCO 3 to BaCO 3 (M 2+ radii : Ca < Sr < Pb < Ba). The CaCO 3 structure is, therefore, the most distorted, whereas the BaCO 3 structure is the least distorted. Several linear structural trends are observed in plots of selected parameters as a function of the unit-cell volume, V. These parameters are radii of the nine-coordinated M 2+ cations, the unit-cell axes, the average and distances, average angle, and aplanarity. These linear trends are the result of the effective size of the divalent ionic radius of the M cations that are coordinated to nine oxygen atoms. The geometrical features of the CO 3 group can be obtained reliably only by using neutron-diffraction data, especially in the presence of other heavy atoms.Keywords: CaCO 3 , aragonite, SrCO 3 , strontianite, PbCO 3 , cerussite, BaCO 3 , witherite, Rietveld refinements, synchrotron highresolution powder X-ray diffraction (HRPXRD), neutron diffraction, crystal structure. SOMMAIreNous avons affiné la structure cristalline de quatre carbonates orthorhombiques isostructuraux, CaCO 3 (aragonite), SrCO 3 (strontianite), PbCO 3 (cérusite), et BaCO 3 (witherite), par la méthode de Rietveld en utilisant les données acquises par diffraction X à haute résolution avec rayonnement synchrotron. Pour le BaCO 3 , nous avons utilisé des données obtenues par diffraction neutronique, aussi affinées par la méthode de Rietveld. Pour l'aragonite, nous avons affiné la structure par diffraction X sur monocristal. Ces carbonates répondent au groupe spatial Pmcn, avec Z = 4. Le groupe CO 3 est légèrement non planaire, et les deux distances C-O indépendantes diffèrent légèrement. Le groupe CO 3 devient plus symétrique et moins aplanaire en allant de CaCO 3 à BaCO 3 (M 2+ rayon : Ca < Sr < Pb < Ba). La structure de CaCO 3 serait donc la plus difforme, tandis que la structure de BaCO 3 serait la moins difforme. Plusieurs tendances structurales sont évidentes dans les tracés de paramètres choisis en fonction du volume de la maille élémentaire, V. Parmi ces paramètres sont le rayon des cations M 2+ à coordinence neuf, les dimensions le long des axes de la maille élémentaire, les distances et moyennes, l'angle moyen, et l'aplanarité. Ces tracés linéaires sont le résultat de la dimension effective du rayon ionique du cation M bivalent coordonné aux ne...

Structure D 2000The Structure of Ferrihydrite, a Nanocrystalline Material. -The structure of the title compound, Fe 10 O 14 (OH) 2 , is determined by real-space modeling of the pair distribution function derived from direct Fourier transformation of the total X-ray scattering. The compound crystallizes in the hexagonal space group P63mc. In its ideal form the structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis δ-Keggin cluster. -(MICHEL*, F. M.; EHM, L.; ANTAO, S. M.; LEE, P. L.; CHUPAS, P. J.; LIU, G.; STRONGIN, D. R.; SCHOONEN, M. A. A.; PHILLIPS, B. L.; PARISE, J. B.; Sci.

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