Vibrational spectroscopic investigation of structurally-related LiFePO4, NaFePO4, and FePO4 compounds

Vibrational spectroscopy was utilized to investigate the local structure of LiFePO4, NaFePO4, and FePO4. The factor group splitting of the intramolecular PO43− vibrations is between 10 and 20 cm−1 less for NaFePO4 than for LiFePO4. This is because Li+ ions have a higher charge density than Na+ ions and can form stronger coordinative bonds with the PO43− anions. Thus, the internal modes are more perturbed in LiFePO4 and exhibit larger factor group splitting effects. The similarity of the factor group multiplets for both LiFePO4 and NaFePO4, particularly the PO43− bending modes, strongly suggests that the 506 and 470 cm−1 bands of LiFePO4 consist almost entirely of lithium translatory motion. There are marked differences between the vibrational spectrum of FePO4 and those of LiMPO4 (M = Mn, Fe, Co, or Ni) or NaFePO4. The monovalent cations interact with the oxygen atoms of the phosphate groups, affecting the frequencies and intensities of the intramolecular PO43− modes, in a manner that is absent in FePO4.