Ionic mobility in Nasicon-type LiM IV 2(PO 4 ) 3 materials followed by 7 Li NMR spectroscopy.

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Ionic mobility in Nasicon-type LiMIV2(PO4)3 materials followed by 7Li NMR spectroscopy. K. Arbi1, 2, I. Sobrados1, M. Hoelzel3, A. Kuhn4, F. Garcia-Alvarado4 and J. Sanz1 1

Instituto Ciencia de Materiales de Madrid (CSIC), 28049 Cantoblanco, Madrid, Spain. Instituto de Ciencia de la Construcción Eduardo Torroja (CSIC), 28033 Madrid, Spain. 3 Forschungsneutronenquelle Heinz-Maier-Leibnitz (FRM II), Technische Universität München, Lichtenbergstr. 1, D-85747 Garching, Germany. 4 Universidad SanPablo-CEU, Montepríncipe, 28668 Boadilla del Monte, Madrid, Spain. 2

Abstract Lithium mobility in LiM2(PO4)3 compounds, with M= Ge, Ti, Sn, Zr and Hf, has been investigated by 7Li Nuclear Magnetic Resonance (NMR) spectroscopy in the temperature range 100-500 K. From the analysis of 7Li NMR quadrupole interactions (CQ and η parameters), Li sites occupancy and exchange processes between structural sites have been studied. Below 250K, Li ions are preferentially located at M1 sites in rhombohedral phases, but occupy M12 sites in triclinic ones. At increasing temperatures, Li mobility has been deduced from spin-spin ( T2−1 ) and spin-lattice relaxation ( T1−1 ) rates. In this analysis, the presence of two relaxation mechanisms in T1−1 plots has been associated with departures of conductivity from the Arrhenius behavior. At high temperatures, residence times at M1 and M12 sites become similar and conductivity significantly increase. This superionic state can be achieved by enlarged order-disorder transformations in rhombohedral phases, or by sharp first order transitions in triclinic ones. Results described in the LiTi2(PO4)3 sample have been compared with those obtained in rhombohedral Li1+xTi2-xAlx(PO4)3 and LiTi2-xZrx(PO4)3 series showing respectively higher and lower conductivities. In the case of Li1.2Ti1.8Al0.2(PO4)3, displaying the highest reported conductivity, NMR results are discussed in relation with those obtained by Neutron Diffraction (ND) and Impedance Spectroscopy (IS). Diffusion coefficients determined by NMR Pulse Field Gradient (PFG) technique are similar to those deduced from Impedance Spectroscopy and NMR relaxation data. Introduction Much effort has been devoted during the last few decades to understand Li dynamics in LiM2(PO4)3 compounds with NASICON structure, because its potential application as electrolyte or intercalation electrode in solid electrochemical devices[1-3]. The framework of NASICON-type materials is built up by M2(PO4)3 units, in which two MO6 octahedrons are linked to three PO4 tetrahedrons by sharing oxygens. Tetrahedra and octahedra of contiguous M2(PO4)3 units are bounded to form the threedimensional network of Nasicon phases (Figure 1a). The usual symmetry of LiM2(PO4)3 compounds is rhombohedral R3c , although in some cases a triclinic distortion (space group C 1 ) has been found. In rhombohedral Ge and Ti samples, Li+ ions are surrounded by six oxygens at M1 sites[4,5]; but, in triclinic Sn, Zr and Hf phases, Li+ ions are coordinated to four oxygens at M12 sites located between M1 and M2