Origin of valence and core excitations inLiFePO4 andFePO4

Abstract

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 ofFePO4 are characterized by interband transitions found between 0 and 20 eV, which are not observed inLiFePO4. Spectra are fully analysed using band structure calculations and calculateddielectric functions. In particular, we show that interband transitions observed inFePO4 spectra originate from the states at the top of the valence band, which have mainlyoxygen p character. From core-loss EELS, it is observed that the O-K edge inFePO4 has apre-edge peak below the threshold of the main O-K edge. This pre-edge peak is not observed in the O-Kspectra of LiFePO4. The position of the pre-edge peak is determined by a charge transfer process, whichshifts the position of the iron 3d bands with respect to the conduction band. Theintensity of the pre-edge peak is also determined by the changes in the hybridizationof iron 3d and oxygen states as a result of extraction of lithium ions from theLiFePO4 lattice. We show that the extraction of lithium ions fromLiFePO4 results in large changes in the electronic structure, such thatFePO4 can be considered to be a charge transfer insulator whileLiFePO4 is a typical Mott–Hubbard insulator.