Magnetic Structures of the Triphylite LiFePO₄ and of Its Delithiated Form FePO₄

The magnetic structures of LiFePO4 and of its delithiated form FePO4 (triphylite, olivine group, space group Pnma) have been solved using neutron diffraction on polycrystalline samples. Both compounds show antiferromagnetic behavior below 52 K for LiFePO4 (Fe2+) and below 125 K for FePO4 (Fe3+), characterized by the appearance of extra peaks in the neutron diffraction patterns below the Néel temperature. These magnetic reflections are indexed with a propagation vector k = (0,0,0). The magnetic moments of the four iron atoms present in the unit cell are oriented along [010] for LiFePO4 and mostly along [100] (with a small component along [010]) for FePO4. The magnetic structures and the differences in the Néel temperatures are discussed in terms of super and super-super exchange intractions and of anisotropy of Fe2+. A comparison is made with other olivine compounds with similar cation distribution.