From order to disorder: The structure of lithium-conducting garnets Li 7 − x La 3Ta xZr 2 − x O 12 (x = 0–2)

Abstract

Structural properties of Li 7 − x La 3Ta xZr 2 − x O 12 garnets with x = 0–2 were clarified by means of Rietveld analysis using results of X-ray diffraction and neutron diffraction at room temperature and at low temperature. In this work the controversy between Awaka [1] and Murugan [2] concerning the crystal structure of Li 7La 3Zr 2O 12 was solved. It was shown that the tetragonally derived garnet structure of space group I4 1/ acd described by Awaka [1] is the thermodynamically stable structure for Li 7La 3Zr 2O 12. In the three-dimensional sub-network of this structure, lithium is ordered and occupies all octahedral sites as well as one third of the tetrahedral sites. Li 7 − x La 3Ta xZr 2 − x O 12 garnets with x = 0.125–2 crystallize in the garnet structure, space group Ia 3 ¯ d. As the tantalum content increases, the lattice parameter at room temperature decreases from a = 12.9833(1) Å for Li 6.875La 3Ta 0.125Zr 1.875O 12 down to a = 12.81224(7) Å for Li 5La 3Ta 2O 12. In Li 6.5La 3Ta 0.5Zr 1.5O 12 garnet, lithium atoms are statistically partitioned among octahedral sites (occ.: 0.80(2)) and tetrahedral sites (occ.: 0.56(4)). In the cases of ordered Li 7La 3Zr 2O 12 tetragonally derived garnet and statistically disordered Li 6.5La 3Ta 0.5Zr 1.5O 12 garnet, lithium partitioning remains unchanged as temperature decreases.