Temperature dependent valence states and magnetic properties of lithium delithiated Li0.59FePO4

Polycrystalline samples of Li0.59FePO4 were prepared by oxidation reaction of pure LiFePO4, and temperature dependent valence states and magnetic properties were studied. X-ray diffraction patterns showed a biphasic olivine-type orthorhombic structure (space group: Pnma) where triphylite (LiFePO4) and heterosite (FePO4) coexisted. Also, we have observed the biphasic antiferromagnetic ordering of triphylite and heterosite with different antiferromagnetic to paramagnetic ordering transition temperatures. Mössbauer spectra of Li0.59FePO4 showed a two-phase asymmetrical eight line pattern due to the different electric quadrupole interactions in triphylite and heterosite. The iron ions of triphylite and heterosite are at Fe2+ (3d6) and the Fe3+ (3d5) valence state. Also the large value of ΔEQ for the triphylite phase was originated from the asymmetric charge distribution of FeO6. These results indicate that the charge distributions around the Fe nucleus in the triphylite are more asymmetric because of the contribution from the crystal field and the ion valence state.