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 conduction channels of the NASICON framework has been determined from electrical conductivity measurements in samples of composition LiM2(PO4)3 and LiMM‘(PO4)3, where M and M‘ are Ge, Ti, Sn, and Hf, all compounds belonging to space group R3c. Two lithium sites, M1 and M2, inside the channels, can be distinguished. The sites are connected through triangular bottlenecks of oxygen atoms, and the size of the bottleneck has been estimated from refined and simulated structures for each composition. The plot of activation energy vs bottleneck size shows two regimes:  for sizes up to 2.04 A the activation energy decreases steeply, but above 2.04 A the activation energy is almost constant. These regimes are discussed on the basis of the effective Li+ ionic radius for the compounds analyzed.