An ab initio model of electron transport in hematite (α-Fe2O3) basal planes

Abstract

Transport of conduction electrons through basal planes of the hematite lattice was modeled as a valence alternation of iron cations using ab initio molecular orbital calculations and electron transfer theory. A cluster approach was successfully implemented to compute electron-transfer rate-controlling quantities such as the reorganization energy and electronic coupling matrix element. Localization of a conduction electron at an iron lattice site is accompanied by large iron–oxygen bond length increases that give rise to a large internal component of the reorganization energy (1.03 eV). The internal reorganization energy calculated directly is shown to differ from Nelsen’s four-point method due to the short-range covalent bridge interaction between the Fe–Fe electron transfer pair in the hematite structure. The external reorganization energy arising from modification of the lattice polarization surrounding the localization site is predicted to contribute significantly to the total reorganization energy. The interaction between the reactants and products electronic states near the crossing-point configuration is 0.20 eV and is consistent with an adiabatic electron-transfer mechanism. Electron transfer is predicted to possess a small positive activation energy (0.11 eV) that is in excellent agreement with values deduced from conductivity measurements. Measured electron mobility can be explained in terms of nearest-neighbor electron hops without significant contribution from iron atoms further away. Comparison of the predicted maximum polaron binding energy with the predicted half bandwidth indicates compliance with the small-polaron condition. Therefore the localized electron treatment is appropriate to describe electron transport in this system.