Electron transfer calculations between edge sharing octahedra in hematite, goethite, and annite

Abstract

A key reaction underlying the charge transport in iron containing oxides, clays, micas is the Fe2+-Fe3+ exchange reaction between edge-sharing iron octahedra. These reactions facilitate conduction in these minerals by the thermally-activated hopping of small polarons across the lattice. Depending on the mineral and local charge state the small polaron can either encase an electron or hole. The probability for conduction of small polarons depends strongly on the height and adiabicity of the reaction barrier, with larger and more diabatic barriers yielding slow conduction associated with either weak coupling or a large prerequisite rearrangement of the lattice during charge transport. To model these reactions, a first principle electron transfer (ET) method was developed to model the small polaron hopping between the edge-sharing octahedra sites in hematite (e- polaron), goethite (e- polaron), and annite (h+ polaron) bulk structures. The ET method is based on electronic structure methods (i.e., plane-wave Density Functional Theory) capable of performing calculations with periodic cells and large size systems efficiently while at the same time being accurate enough to be used in the estimation of the electron-transfer coupling matrix element, VAB, and the electron transfer transmission factor, κel. The calculations confirmed the existence of small polarons in all three minerals, and the reactions were predicted to be strongly adiabatic. It was found that transfer of a hole in the octahedral layer of annite had an adiabatic barrier of 0.311 eV, and the transfer of an extra electron in hematite and goethite had adiabatic barriers of 0.242 eV and 0.232 eV respectively. The electronic coupling parameters,VAB, were found to be 0.188 eV, 0.196 eV, and 0.102 eV respectively for hematite, goethite, and annite. While similar bonding topologies pertain, the findings reveal the importance of subtle differences in local structure.