Synthesis and ionic conductivity of new high Li ion content garnets, LnSr2Ta2Li7O12 (Ln = La, Pr, Nd, Sm, Gd)

In this paper we report the synthesis and Li ion conductivity of the new high Li content garnet phases LnSr2Ta2Li7O12 (Ln = La, Pr, Nd, Sm or Gd). Close inspection of the X-ray diffraction patterns indicate that these systems are mostly composed of the ordered tetragonal garnet phase, along with a small amount of cubic garnet. The presence of a small amount of cubic garnet phase can most likely be correlated with partial Li loss on synthesis as well as some degree of H+/Li+ exchange. The latter was supported by results from variable temperature X-ray diffraction studies and thermogravimetric analysis. The adoption of the tetragonal garnet structure by these new systems highlights further that in order to accommodate 7 Lithium ions within the garnet structure, then cation ordering must occur to prevent short Li-Li interactions. In line with other tetragonal garnet systems, the Li ion conductivity is shown to be low, as a result of this ordered Li distribution.