Crystal Structure Revision and Identification of Li+Ion Migration Pathways in the Garnet-like Li5La3M2O12 (M = Nb, Ta) Oxides

Bond valence sums for the ion positions in singlecrystal structure data of the garnet-like fast lithium ion conductors Li5La3M2O12 (M = Nb, Ta) exhibit unusually large deviations from the ideal valences. The root-mean-square bond valence mismatch (commonly termed "global instability index" GII) and the chemical plausibility of the structure model can be significantly improved by optimizing the light atoms (oxygen and lithium) positions using a bond valence mismatch minimization procedure in the previously suggested space group I213 or its centrosymmetric counterpart Ia. Possible pathways for lithium ion migration in Li5La3M2O12 are identified by a bond valence analysis. Li-bond valence mismatch isosurface models for Li+-ion transport pathways are found to be nearly the same in both compounds Li5La3Nb2O12 and Li5La3Ta2O12. The characteristic feature of the three -dimensional Li+-ion pathway network is a nonplanar square of partially occupied Li sites.