Local Structure of Ferrihydrite and Feroxyhite by Exafs Spectroscopy

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Abstract
Synthetic 2-line and 6-line ferrihydrite and feroxyhite samples prepared from ferric salt solutions have been investigated by EXAFS spectroscopy. All these materials have been found to be short-range ordered, consisting of Fe octahedra linked by comers, edges, and faces. Their local structures are related to those of well-crystallized (oxyhydr)oxides, and the absence of hkl reflections in some samples is attributed to the small size of coherent scattering domains. The presence of face sharings indicates that these materials have structural similarities with hematite. Based on Fe-Fe distances and the analysis of the static disorder, it has been concluded that the local structure of feroxyhite is close to that of hematite, whereas ferrihydrite has common structural features with both hematite (αFe203) and cdβFeOOFI. The local structure of ferrihydrite thus differs from that of aqueous Fe polymers obtained by the partial hydrolysis of ferric nitrate and chloride solutions. Differences of local structures among hydrous Fe oxides and aqueous polymers have been interpreted on the basis of a room temperature stability phase diagram established for well-crystallized (oxyhydr)oxides.