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 (0.5LiFePO(4) + 0.5FePO(4); 0.75LiFePO(4) + 0.25FePO(4)) 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 similar to(0.64)FePO(4) and LiFePO4 + Li similar to(0.64)FePO(4), 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 Li-O bonds are longer than in LiFePO4, whereas Fe-O 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. (c) 2005 Published by Elsevier SAS.