The electronic properties of chemically delithiated Li_xFePO_4

We report on the structure and the electronic features of LiFePO4 and the delithiated mixture (1-x)Li_1FePO_4 + x Li_0FePO_4 powder obtained by chemical delithiation. X-ray diffraction, Fourier transform infrared spectroscopy, and magnetic experiments are analyzed for x = 0.0, 0.5, and 1.0. The magnetic properties give evidence of a super-paramagnetism associated to iron-based ferromagnetic nanoparticles, superposed to the intrinsic magnetic contribution of LiFePO_4 which undergoes an antiferromagnetic ordering at T_N(x=1)=50 K and T_N(x=0)=125 K. Magnetization curves show superparamagnetism due to nanometer- sized magnetic particles. The magnetic clustering (amount of iron in the ferromagnetic nanoparticles, mean size distribution) is independent of the lithium concentration.