Local Structure and Magnetic Properties of Li x FePO 4 Glasses
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Local Structure and Magnetic Properties of LixFePO4 Glasses Pawel Jozwiak1, Jerzy Garbarczyk1, François Gendron2, Alain Mauger3, and Christian M. Julien2 1 Faculty of Physics, Warsaw University of Technology, Koszykowa 75, Warsaw, 00662, Poland 2 INSP, University Paris 6, 140 rue de Lourmel, Paris, 75015, France 3 MPPU, CNRS, 140 rue de Lourmel, Paris, 75015, France ABSTRACT A series of LixFePO4 samples (0≤x≤1) have been synthesized in the vitreous state by fast quenching technique. Characterization methods include XRD, TG, FTIR, magnetic susceptibility and electrical measurements. FTIR studies reveal that in the entire composition range the local structure is identical to the olivine lattice including PO43- oxo-anions linked to FeO6 octahedra. Magnetic susceptibility data show that the antiferromagnetic ordering does not exist so far and the paramagnetic behaviour follows the Curie-Weiss law above 30 K. Both the Curie parameter and the Weiss temperature suggest a strong weakness of the superexchange interactions that are consistent with the analysis of infrared spectra. Electrical conductivity shows an activated conduction mechanism with Ea=0.67 eV. INTRODUCTION The olivine-like compound LiFePO4 has been studied as the positive electrode-active material in lithium rechargeable batteries because of its low cost, low toxicity, and relatively high theoretical specific capacity of 170mAh/g [1-5]. LiFePO4 belongs to the olivine family of lithium orthophosphates with space group Pnma [6]. It has an elongated a axis, of length that is about twice the b and c cell constants, with a structure that is comprised of nearly close-packed oxygen atoms in hexagons, with Li and Fe ions located at the center of octahedral sites and P ions situated in tetrahedral sites. The FeO6 octahedra are corner shared and cross-linked with the PO4 tetrahedra, forming a three-dimensional network, with tunnels along the [010] and [001] directions occupied by Li ions. While the physical and chemical features of crystalline olivine-like LiFePO4 are of current interest, phosphate glasses have unique properties which make them useful for a wide range of technical applications. In this respect, numerous works have been devoted to the study of P2O5-based glasses for instance. General trends exhibit conductivity enhancement related to the structural change of oxo-anions forming the framework structure of the glasses [7-8]. However, to our knowledge the properties of amorphous LiFePO4 have not been reported so far. Following our previous works related to polyphosphate-like glasses such as Li3Fe2(PO4)3, Li5V2(PO4)5, Li6Mo2(PO4)6 [9-10], we prepared vitreous LixFePO4 with various lithium concentrations in the entire range 0≤x≤1. In this work, we investigate the local structure and magnetic properties of LixFePO4 glasses prepared by fast-quenching method. Characterizations include X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and magnetometry. The structural changes in the phosphate matrix may reflect the role of loca
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