Short- to Medium-Range Atomic Order and Crystallite Size of the Initial FeS Precipitate from Pair Distribution Function Analysis

Pair distribution function analysis (PDF) of X-ray diffraction data, collected at 11-IDC and 1-ID at the Advanced Photon Source, provides the atomic structure and primary crystallite size of FeS both freshly precipitated (FeS fresh ) and aged (FeS aged ). The short- to medium-range structure of both FeS fresh and FeS aged are nearly identical with that of highly crystalline (bulk) mackinawite. Attenuation in the observed range of structural coherence of the PDF for FeS fresh indicates an average crystallite size on the order of ∼2 nm. This range of structural coherence increased with aging of the sample under hydrothermal conditions due to growth of the individual crystallites, although the mechanism by which this growth occurs is not clear at present. Electron microscopic imaging confirms the presence of individual nanoscale crystallites and provides some insight into their aggregation behavior as larger clusters. The initial, fresh precipitate does not exhibit long-range atomic structure because it is nanocrystalline. The so-called X-ray amorphous nature of FeS fresh is the result of the limited range of structural coherence imposed by the size of the individual crystallites rather than the result of a lack of medium- and long-range atomic order. We propose that the discrepancies in the literature over crystallite size and the atomic structure of FeS fresh are due primarily to the varying degrees of aggregation of uniformly distributed and nanocrystalline FeS particles.