Lithium Mobility in Li_1.2Ti_1.8R_0.2(PO₄)₃ Compounds (R = Al, Ga, Sc, In) as Followed by NMR and Impedance Spectroscopy

Composition and lithium mobility have been analyzed in Li 1+ x Ti (PO 4 ) 3 compounds ( x = 0.2 and R 3+ = Al 3+, Ga 3+, Sc 3+, and In 3+ ) by NMR and impedance spectroscopy. All of the compounds display the rhombohedral symmetry, and in all cases the trivalent cations are incorporated into the NASICON framework. From the analysis of the quadrupole constant ( C Q ) and spin−spin relaxation rate ( T 2 - 1 ) deduced from 7 Li NMR spectra of Li 1.2 Ti 1.8 Al 0.2 (PO 4 ) 3, two regimes associated with local and long-range motions of lithium have been identified. From the analysis of the spin-lattice relaxation rate ( T 1 -1 ) as a function of reciprocal temperature, the residence times of lithium at structural sites have been estimated and their dependence on temperature have been analyzed. Differences in activation energy deduced from NMR (0.22 eV) and conductivity (0.28 eV) measurements are discussed in terms of correlations in lithium motion that seem to decrease at increasing temperature.