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+xLn3-xAxM2O12 (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.

A comparison of the transport properties of lithium-stuffed garnets and the conventional phases Li3Ln3Te2O12

“…The complex lithium distribution across multiple sites within the oxide octahedron as well as the tetrahedron has been consistently reported for the related Sr 2 + phases [11,16] as well as the series Li 5 + x Ba x La 2 Ta 2 O 12 [4]. At higher lithium contents it is known that Li + -ordering can occur [6,7], and extremely highresolution neutron diffraction data have revealed possible twophase behaviour [8]. However, neither effect was evident in any of the data collected from the present compounds.…”

“…All of the peaks could be indexed in a straightforward manner with no evidence of either a tetragonal distortion that has been observed for compositions Li 7 La 3 M 2 O 12 (M ¼Zr, Sn) [6,7] or the peak asymmetry arising from a mixture of garnet phases seen in samples of Li 5 Nd 3 Sb 2 O 12 [8]. The structural model was refined in a straightforward manner.…”

“…These lithium-stuffed garnets house the lithium in a mixture of the tetrahedrally coordinated B site and in a large, heavily distorted octahedron that is vacant in the classical A 3 B 3 C 2 O 12 compounds. There is considerable positional and occupational disorder in the fast-ion conducting phases [3,4] and the conductivity shows a general increase with lithium content [5] until for compositions such as Li 7 La 3 Sn 2 O 12 lithium cation ordering occurs and the conductivity is drastically reduced compared to the disordered phases [6,7].…”

Phase formation of a garnet-type lithium-ion conductor Li7−3Al La3Zr2O12

“…However, the ionic conductivity of these materials was low at around 10 − 6 S cm − 1 at 25°C. Therefore, extensive studies have been pursued to improve the ionic conductivity of these systems [6][7][8]. Garnet-like Li 7 La 3 Zr 2 O 12 (LLZ) was discovered by Murugan et al [8] and exhibits a high bulk conductivity of ca 5 × 10 −4 S cm −1 at 25°C in addition to electrochemical stability with lithium metal.…”

Nanostructured Garnet-Type Solid Electrolytes for Lithium Batteries: Electrospinning Synthesis of Li7La3Zr2O12 Nanowires and Particle Size-Dependent Phase Transformation

“…7 The tetragonal distortion is a result of Li + ordering (blue spheres in Figure 1b) that eliminates the short Li -Li interactions and disordered Li + clustering that is responsible for the high conductivity in the HT-cubic phase. 4,[8][9][10][11] Through the development of low temperature synthesis methods for LLZO, a third phase with cubic structure has emerged. 12,13 This so-called low temperature cubic phase (LT-cubic) has the same structure as HT-cubic, but with a slightly larger lattice constant and lower ionic conductivity than even t-LLZO.…”