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.