Origin of valence and core excitations in LiFePO4 and FePO4

Electronic structures of LiFePO4 and FePO4 have been investigated using valence and core electron energy loss spectroscopy (EELS) supported by ab initio calculations. Valence electron energy loss spectra of FePO4 are characterized by interband transitions found between 0 and 20 eV, which are not observed in LiFePO4. Spectra are fully analysed using band structure calculations and calculated dielectric functions. In particular, we show that interband transitions observed in FePO4 spectra originate from the states at the top of the valence band, which have mainly oxygen p character. From core-loss EELS, it is observed that the O-K edge in FePO4 has a pre-edge peak below the threshold of the main O-K edge. This pre-edge peak is not observed in the O-K spectra of LiFePO4. The position of the pre-edge peak is determined by a charge transfer process, which shifts the position of the iron 3d bands with respect to the conduction band. The intensity of the pre-edge peak is also determined by the changes in the hybridization of iron 3d and oxygen states as a result of extraction of lithium ions from the LiFePO4 lattice. We show that the extraction of lithium ions from LiFePO4 results in large changes in the electronic structure, such that FePO4 can be considered to be a charge transfer insulator while LiFePO4 is a typical Mott-Hubbard insulator.