Different samples of the sodium-vanadium fluorophosphate cathodic materials have been synthesized via the hydrothermal method, varying the type and content of carbon used in the synthesis. Structural characterization of the composites was performed by powder X-ray diffraction. Magnetic susceptibility measurements and EPR (Electron Paramagnetic Resonance) polycrystalline spectra indicate that some of the samples exhibit V 3+ /V 4+ mixed valence, with the general formula Na 3 V 2 O 2x (PO 4 ) 2 F 3À2x where 0 # x < 1. The morphology of the materials was analyzed by Transmission Electron Microscopy (TEM). A correlation between the type and content of carbon with the electrochemical behavior of the different samples was established. Electrochemical measurements conducted using Swagelok-type cells showed two voltage plateaux at 3.6 and 4.1 V vs. Na/Na + . The best performing sample, which comprised a moderate percentage of electrochemical grade carbon as additive, exhibited specific capacity values of about 100 mA h g À1 at 1C (z80% of theoretical specific capacity). Cyclability tests at 1C proved good reversibility of the material that maintained 98% of initial specific capacity for 30 cycles.
Neutron diffraction and magnetic susceptibility studies of (Li 1Ϫ3x Fe x )NiPO 4 (xϭ0 and xϭ0.033) compounds reveal remarkable differences between the magnetic properties of pure LiNiPO 4 and those of its lightly iron-doped derivative. The spin system associated with the Ni 2ϩ ions (Sϭ1) in the pure compound undergoes a collinear antiferromagnetic ordering at T N ϭ19.1Ϯ0.5 K, with the characteristics of weakly coupled twodimensional ͑2D͒ Ising square planes. By contrast, randomly intercalated iron spins ͑in Li sites͒ between Ni 2ϩ planes comprise a spin-glass-like subsystem which, despite their minute amount, drives the antiferromagnetic transition to higher temperatures T N ϭ25.2Ϯ0.5 K, and significantly modifies the critical behavior of the 2D Ni 2ϩ system. It is argued that the doped compound can serve as a model system for studying the randomly coupled planar Ising model. ͓S0163-1829͑99͒15425-0͔
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.