Cation ordering in Li containing garnets: synthesis and structural characterisation of the tetragonal system, Li7La3Sn2O12

Abstract

In this paper we report synthesis, conductivity and structural data for the novel tetragonal garnet-related system, Li₇La₃Sn₂O₁₂. Neutron diffraction data shows that the tetragonal distortion is related to ordering of Li in three sites within the structure to ensure no short Li–Li interactions. Consistent with the ordered nature of the Li ions, the conductivity is low, with a high activation energy. The results are relevant to related highly conducting cubic garnets, Li₅₊xLn_3−xA_xM₂O₁₂ (Ln = rare earth, A = alkaline earth; M = Nb, Ta, Sb), showing how a high Li content can be accommodated by Li ordering within the garnet structure, supporting previous suggestions by Cussen for the cubic garnets, who proposed the presence of local ordering/clustering of Li in tetrahedral and “octahedral” sites to limit unfavourable short Li–Li interactions.