Thermally-induced cation disorder in LiFePO4

LiFePO4 has a fully ordered olivine structure in samples prepared by solid state reaction below ~ 800 °C but, with increasing temperature, a small amount of Li and Fe site exchange occurs reaching a value of about 4% just below melting at 975 °C. The disorder is reversible on annealing at lower temperatures and is detected by changes in lattice parameters and in cation site occupancies obtained by Rietveld refinement of X-ray powder diffraction data.