Effect of structure on the Fe3+/Fe2+ redox couple in iron phosphates

To understand the role of structure on the position of the octahedral Fe3+/Fe2+ redox couple in compounds having the same polyanions, four iron phosphates: Li3Fe2(PO4)(3), LiFeP2O7, Fe-4(P2O7)(3), and LiFePO4 were investigated. They vary in structure as well as in the manner in which the octahedral iron atoms are linked to each other. The Fe3+/Fe2+ redox couple in the above compounds lies at 2.8, 2.9, 3.1, and 3.5 eV, respectively, below the Fermi level of lithium. The reason for the difference in the position of the redox couples is related to changes in the P-O bond lengths as well as to changes in the crystalline electric field at the iron sites.