Combined Experimental and Computational Analyses on the Electronic Structure of Alluaudite-Type Sodium Iron Sulfate

Alluaudite-type sodium iron sulfate, Na2.56Fe1.72(SO4)(3), has the highest redox potential in any Fe3+/Fe2+ environment; here, its electronic structure is investigated by combining X-ray absorption spectroscopy (XAS), semiempirical ligand field multiplet (LFM) simulations, and ab initio calculations. The LFM simulations for the Fe L-2,L-3-edge XAS reveals the exceedingly ionic character of the Fe-O bond due to the strong inductive effect of the sulfate group; this is one of the main factors which enhance the electrode potential of Na2.56Fe1.72(SO4)(3). Both experimental XAS measurements and ab initio calculations indicate electrochemical charging (desodiation) gives more hybridized iron 3d and oxygen 2p states. The degree of covalency between iron and oxygen changes reversibly along with the change of the iron valence state.