RELATIONSHIP BETWEEN ACTIVATION-ENERGY AND BOTTLENECK SIZE FOR LI+ ION CONDUCTION IN NASICON MATERIALS OF COMPOSITION LIMM'(PO4)(3), M, M' = GE, TI, SN, HF

The activation energy involved in the motion of Li+ ions along the con duction channels of the NASICON framework has been determined from ele ctrical conductivity measurements in samples of composition LiM2- (PO4 )(3) and LiMM'(PO4)(3), where M and M' are Ge, Ti, Sn, and Hf, all com pounds belonging to space group R (3) over bar c. Two lithium sites, M 1 and M2, inside the channels, can be distinguished. The sites are con nected through triangular bottlenecks of oxygen atoms, and the size of the bottleneck has been estimated from refined and simulated structur es for each composition. The plot of activation energy vs bottleneck s ize shows two regimes: for sizes up to 2.04 Angstrom the activation en ergy decreases steeply, but above 2.04 Angstrom the activation energy is almost constant. These regimes are discussed on the basis of the ef fective Li+ ionic radius for the compounds analyzed.