Résumé
The Fe-57 Mossbauer isomer shift and quadrupole splitting of LiFePO4 (LFP) and FePO4 (FP) were evaluated from density functional theory. The effects of structural optimization, Fe 3d electron correlation, and spin configuration are found to be rather small for the isomer shift but not for the quadrupole splitting. The observed decrease of the isomer shift from LFP to FP is explained from the variations of both Fe 4s and Fe 3d electrons and is related to differences in the Fe-O chemical bond properties caused by oxidation of high-spin Fe2+ into high-spin Fe3+. The values of the quadrupole splitting of the two compounds strongly increase from nonspin-polarized to spin-polarized calculations and then slightly increase by taking into account the Fe 3d electron correlation, providing a good agreement with experimental data. The observed decrease of the quadrupole splitting from LFP to FP is quantitatively correlated to the decrease of the magnitude of the highest principal value V-zz of the electric field gradient (EFG) tensor arising from the anisotropy of Fe p-type electrons for FP and Fe 3d(down arrow) elections for LFP. The EFG of FP reflects the distortion of the FeO6 octahedra mainly arising from edge sharing PO4 tetrahedra, while it is due to high-spin Fe2+ for LFP. Finally, the calculations predict that the sign of Vzz changes from positive for LFP to negative for FP, reflecting electron depletion and accumulation, respectively, along the principal axis Z due to the rotation of the principal axis system. As a consequence, the charge discharge cycles of LFP-based cathode for Li-ion batteries produce strong periodic variations of Vzz through almost the overall EFG scale for iron compounds.