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 i ron phosphates: Li3Fe2(PO4)(3), LiFeP2O7, Fe-4(P2O7)(3), and LiFePO4 w ere investigated. They vary in structure as well as in the manner in w hich 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 d ifference in the position of the redox couples is related to changes i n the P-O bond lengths as well as to changes in the crystalline electr ic field at the iron sites.