Vibrational spectroscopic investigation of structurally-related LiFePO 4, NaFePO 4, and FePO 4 compounds

Abstract

Vibrational spectroscopy was utilized to investigate the local structure of LiFePO 4, NaFePO 4, and FePO 4. The factor group splitting of the intramolecular PO 4 3− vibrations is between 10 and 20 cm −1 less for NaFePO 4 than for LiFePO 4. This is because Li + ions have a higher charge density than Na + ions and can form stronger coordinative bonds with the PO 4 3− anions. Thus, the internal modes are more perturbed in LiFePO 4 and exhibit larger factor group splitting effects. The similarity of the factor group multiplets for both LiFePO 4 and NaFePO 4, particularly the PO 4 3− bending modes, strongly suggests that the 506 and 470 cm −1 bands of LiFePO 4 consist almost entirely of lithium translatory motion. There are marked differences between the vibrational spectrum of FePO 4 and those of LiMPO 4 (M = Mn, Fe, Co, or Ni) or NaFePO 4. The monovalent cations interact with the oxygen atoms of the phosphate groups, affecting the frequencies and intensities of the intramolecular PO 4 3− modes, in a manner that is absent in FePO 4.