Lookup NU author(s): Dr Anke Neumann
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Fe-bearing clay minerals serve as an important source and sink for electrons in redox reactions in various subsurface geochemical environments. We apply first-principles calculations using a small polaron hopping approach and Marcus electron transfer theory to examine electron exchange mobilities in an Fe-rich smectite, nontronite Fe2Si4O10(OH)(2). GGA+U calculations provide rates of electron hopping that agree very well with values deduced from variable-temperature Mossbauer data (Schaefer et al. Environ. Sci. Technol. 2011, 45, 540), indicating a surprisingly fast electron mobility at room temperature. Evaluation of the electron transfer (ET) rates within the Hartree-Fock cluster framework for the Fe2+/Fe3+ electron hopping in tetrahedral (TS) and octahedral sheets (OS), as well as across the sheets (TS-OS), shows that the dominant contribution to the bulk electronic conductivity should come from the ET within the OS. Deprotonation of structural OH groups mediating ET between the Fe ions in the OS is found to decrease the internal reorganization energy and to increase the electronic coupling, whereas protonation (to OH2 groups) has the opposite effect. Our calculations suggest that the major factors affecting ET rates are the nature and structure of the nearest-neighbor local environment and the degree of covalency of the bonds between Fe and ligands mediating electron hops. The generally higher reorganization energy and weaker electronic coupling found in Fe-bearing day minerals lead to electron mobilities much lower than in iron oxides.
Author(s): Alexandrov V, Neumann A, Scherer MM, Rosso KM
Publication type: Article
Publication status: Published
Journal: Journal of Physical Chemistry C
Print publication date: 11/01/2013
Date deposited: 10/03/2014
ISSN (print): 1932-7447
ISSN (electronic): 1932-7455
Publisher: American Chemical Society
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