We report first principles calculations aimed at predicting thermodynamically stable structures of ferrihydrite across a range of possible compositions, water contents, and particle sizes from virtual bulk phases to the nanoscale. Based on an assumed stochiometry for the core of ferrihydrite, Fe5O8H.nH2O, we have used ab initio calculations combined with evolutionary structure searching to find the lowest enthalpy structure depending on the water content. This investigation has generated and evaluated more than 4000 unique configurations. For the Fe5O8H stochiometry a new structure with energetic stability comparable to the akdalaite model proposed by Michel et al.[1] has been found. While the akdalaite model is made of 20% tetrahedral iron, this competitive structure is made of 100% octahedral iron and shows face sharing. Ab initio thermodynamics calculations were performed to determine the relative phase stability between hematite, goethite, the theoretically generated structures, the akdalaite model, and the modified akdalaite model introduced by Manceau et al.[2], as a function of water partial pressure and temperature. Particle size effects were further taken into account using the thermodynamic framework work of T. Hiemstra [3] but by including explicit surface energy calculations. The collective findings suggest a co-existence of select phases is possible depending primarily on water content and particle size. [1] F. M. Michel et al., Science, 316, 1726-1729 (2007) [2] A. Manceau et al., American Mineralogist, 99, 102-108 (2014) [3] T. Hiemstra, Geochimica et Cosmochimica Acta, 158, 179-198 (2015)

Ferrihydrite: Structure possibilities, thermodynamics, and particle size effects from first principles calculations

251st National Meeting (2016)

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