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

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, Li(5+x)Ln(3-x)A(x)M(2)O(12) (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.