High lithium ionic conductivity in the Li1+xAlxGeyTi2-x-y(PO4)3 NASICON series

Two Li1+xAlxGeyTi2-x-y(PO4)(3) (0.2 less than or equal to x less than or equal to 0.8; y = 0.8, 1.0) Solid solutions have been prepared as polycrystalline powders. These compounds crystallize in the NASICON-type structure, R (3) over barc space group, and the crystal structures have been characterized by the Rietveld method with laboratory X-ray powder diffraction data. The cell parameters evolution along the two series agrees with the substitution of larger Ti4+ by smaller Ge4+ and Al3+ cations. The electrical properties have been characterized by an impedance study. Bulk conductivity values at room temperature are close to 10(-3) S.cm(-1) with low activation energies ( approximate to 0.35 eV). The trajectories of the Li+ cations have been simulated from the bond valence sum calculation. Structural keys, which justify the high ionic conductivity and the low activation energy, are discussed.