The characterization of LiFePO4 and of materials synthesized by chemical delithiation of pure LiFePO4 using potassium peroxydisulfate is reported. X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), Electron Paramagnetic Resonance (EPR) spectroscopy and magnetization measurements are analyzed for lithium concentrations x = 0, 0.5, and 1. XRD and EPR data show that the x = 0.5 sample is phase separated into 0.5LiFePO(4) + 0.5FePO(4), in agreement with prior experiments showing that the mixed-valence LixFePO4 compound is unstable. The magnetic properties give evidence of a superparamagnetism associated to iron-based ferromagnetic nanoparticles, superposed to the intrinsic magnetic contribution of the powder which undergoes an antiferromagnetic ordering at T-N(x = 1) = 50 K and T-N(x = 0) = 125 K. The magnetic clustering (amount of iron in the ferromagnetic nanoparticles, mean size distribution) is independent of the lithium concentration. Both the ESR signal of the magnetic clusters and the magnetization curves give evidence of a magnetic interaction between the magnetic clusters and their environment which acts as a friction.