Local structure and lithium mobility in intercalated Li3AlxTi2-x(PO4)3 NASICON type materials: a combined neutron diffraction and NMR study

The structural features of intercalated Li3AlxTi2-x(PO4)(3) compounds, with x = 0 and 0.2, have been deduced by Rietveld analysis of neutron diffraction (ND) patterns recorded between 100 and 500 K. The Li insertion decreases the symmetry from R (3) over barc to R (3) over bar in analyzed compounds. In pristine Li1+xAlxTi2-x(PO4)(3) samples, Li occupies mainly six-fold M-1 sites at ternary axes; but in lithiated Li3AlxTi2-x(PO4)(3) samples, Li is located near M-2 positions at M-3/M-3' four-fold coordinated sites. In both cases. Li arrangement minimizes electrostatic Li-Li repulsions. The insertion of lithium resulted in the reduction of Ti4+ to Ti3+ that shifts Li-7, Al-27 and P-31 MAS-NMR resonances towards more positive chemical shifts, improving the resolution of different sites. The detection of twelve components in Li-7 MAS-NMR spectra recorded at room temperature suggests the location of Li+ ions at three-oxygen faces that define M-2 cavities. From Li-7 MAS-NMR spectra, the occupancy of sites and mobility of lithium were investigated in the temperature range 100-500 K The correlation between structural information, deduced by neutron diffraction, and lithium mobility, deduced by NMR spectroscopy, provides new insights into structural factors that affect lithium mobility in materials with NASICON structure.