Crystal chemistry of the olivine-type LixFePO4 system (0⩽x⩽1) between 25 and 370 °C

The crystal chemistry of two initial mixtures of LiFePO4 and heterosite FePO4 (0.5LiFePO4 + 0.5FePO4; 0.75LiFePO4 + 0.25FePO4) was investigated through Neutron diffraction at 350 and 370 °C, respectively, and at room temperature after cooling. At 350 and 370 °C, Li0.5FePO4 and Li0.75FePO4 are refined as olivine-type single phases, in which Li+ ions are disordered. Significant anisotropic microstrains, within the (001)Pmnb planes, occur, which may be accounted for by heterogeneous distance distributions within and between the [100]Pmnb Li+ channels. On cooling back to room temperature, Li0.5FePO4 and Li0.75FePO4 single phases separate into mixtures of FePO4 + Li∼0.64FePO4 and LiFePO4 + Li∼0.64FePO4, respectively. The Li content of this metastable intermediate phase may correspond to the occupancy of 2 out of 3 Li sites within [100]Pmnb Li+ channels. In Li0,64FePO4, average LiO bonds are longer than in LiFePO4, whereas FeO bond lengths are shortened, due to a fraction of Fe(III). This may be at the origin of the metastability of such intermediate phase, and thus of the two-phase mechanism between LiFePO4 and FePO4.