Unexpected cationic distribution in tetrahedral/octahedral sites in nominal Li1+xAlxGe2−x(PO4)3 NASICON series

Nominal Li1+xAlxGe2−x(PO4)3 (0 ≤ x ≤ 1.2) lithium fast ion conductors have been prepared as polycrystalline powders and characterised by Rietveld analysis of laboratory X-ray powder diffraction patterns (LXRPD). All samples display the ideal R 3¯c symmetry of compounds with NASICON structure. However, 7Li, 27Al, 31P MAS–NMR and AEM analyses suggest an unusual cationic distribution where Al is located in both octahedral and tetrahedral sites, replacing germanium and phosphorus, respectively. In these compounds, Li ions are preferentially disposed at M1 sites, but additional Li incorporated to compensate the charge deficit of aluminium are located near M2 sites of the NASICON structure. Local mobility of Li is responsible for narrowing effects detected in 7Li MAS–NMR experiments. Impedance spectroscopy measurements show that these materials are excellent lithium ionic conductors with bulk conductivity values close to 10− 4 S cm− 1 at room temperature.