Synthesis and structural characterisation of the Li ion conducting garnet-related systems, Li6ALa2Nb2O12 (A=Ca, Sr)

Lithium Ion Conduction in Oxides

“…A structural analysis of Li 6 La 2 SrNb 2 O 12 shows the same complex distribution of lithium across octahedra and tetrahedra but identified a small reduction in the occupancy of the tetrahedra, from 0.59 at room temperature to 0.55 at 350°C. [177] Whilst this change is at the detection limits of the structural refinement, conductivity data show a clear change in the performance of this material depending on the sample history. If the material is quenched from high temperature then the low temperature conductivity is increased by a factor of five compared with a sample that has been slowly cooled to room temperature as shown in Figure 3.29.…”

“…The displacement from the centre of the octahedron reduces four and increases two of the Li-O distances and gives what can be considered as a distorted tetrahedral coordination environment that is a consistent observation in these lithium-stuffed compositions. [173][174][175][176][177] By studying these structures over the temperature range 2 K to 873 K it was established that the lithium distribution is largely temperature independent indicating that the time and spatially averaged structural picture represents static disorder of lithium ions, rather than a dynamic motion of cations inside the octahedron. In addition to (a) the three tetrahedral sites occupied in conventional garnets there are (b) two trigonal prismatic and (c) six large octahedral sites per formula unit that are capable of accommodating Li þ [165,173] The presence of shared polyhedral faces between the oxide tetrahedra and octahedra necessarily leads to the presence of short Li Á Á Á Li distances in the structure.…”

“…[173][174][175]184] Moreover, the signals showed a large variation with temperature that does not reflect the minimal temperature dependence of the lithium distribution observed crystallographically. [173,177] A study of the solid solution that bridges the gap between the insulating Li 3 Nd 3 Te 2 O 12 and the fast ion conducing phase Li 5 La 3 Ta 2 O 12 has identified the pathway for lithium conductivity. [185] On introducing as little as 2% lithium into the oxide octahedra, the conductivity increases by two orders of magnitude compared with Li 3 Nd 3 Te 2 O 12 as shown in Figure 3.27.…”

“…Triangles and circles represent data collected from A ¼ Ca and Sr respectively. Filled and empty circles indicate data collected from samples that were quenched or slow cooled from 700°C [177] history dependence of the conductivity in some of these samples. It is possible that this phenomenon is widespread in the garnets but many of the reported syntheses do not detail how the sample is cooled from the final synthesis temperature and so make it difficult to reliably compare conflicting results.…”

Effect of Sb-Ba codoping on the ionic conductivity of Li7La3Zr2O12 ceramic

“…In addition, our previous work revealed that partially substituting Sb 5 þ for Zr 4 þ introduces a special grain boundary layer, which improves grain boundary and then total conductivity [22]. It is reported that ionic conductivity has been enhanced by increase in the occupancy of Li2 site while reduction in the occupancy of Li1 site through the substitution of divalent alkaline-earth metal ions (Ca 2 þ , Sr 2 þ , Ba 2 þ ) for La 3 þ in garnet structure electrolyte [23,24].…”