HR: 16:00h
AN: V14B-01 INVITED    [Abstracts]
TI: Structure Determination of Nanocrystalline Materials
AU: * Michel, F M
EM: fmichel@ic.sunysb.edu
AF: Stony Brook University, Department of Geosciences 255 ESS Building, Stony Brook, NY 11794-2100, United States
AB: Nanocrystalline materials play an important role in pristine and contaminated geochemical systems. In particular, the reactive surfaces of nanosized ferric and aluminum hydroxides are effective scavengers and transporters of metals and metalloids under aqueous conditions. Establishing the atomic arrangements of these nanocrystalline phases is an essential step towards understanding their structure-property relationships and their roles in geochemical environments. Solving the structures of nanosized materials, often forming with finite particle sizes less than 10 nm, is extremely challenging and often not feasible using conventional structure determination methods. Nanocrystalline ferrihydrite is one example of a material often referred to as being X-ray amorphous because it exhibits poorly defined diffraction maxima in conventional powder patterns. Such features primarily result from the effects caused by disorder and extremely small particle sizes and inhibits quantitative structure determination. This can be avoided by evaluating the atomic arrangement in real-space using total scattering methods combined with pair distribution function (PDF) analysis. PDF analysis is proving to be an essential tool in the study of nanocrystalline materials because it enables the incorporation of both the Bragg and diffuse scattering components and therefore information regarding the short-, intermediate-, and long-range ordering is attainable. We recently used this method to investigate the atomic structures of a number of important nanosized transition metal phases, including ferrihydrite. The initial study of synthetically-derived ferrihydrite resulted in the development of a new structure model for this phase. We are now expanding this investigation by evaluating samples of ferrihydrite occurring in natural settings which are inherently more complex formation environments due to the presence of organics and other metal and metalloid species. The total X-ray scattering method and PDF analysis will be introduced and experimental high-energy scattering data collected at a synchrotron facility will be presented.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting