From order to disorder: The structure of lithium-conducting garnets Li7-xLa3TaxZr2-xO12 (x=0-2)

Structural properties of Li7-xLa3TaxZr2-xO12 garnetswith 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 Li7La3Zr2O12 was solved. It was shown that the tetragonally derived garnet structure of space group 14(1)/acd described by Awaka [1] is the thermodynamically stable structure for Li7La3Zr2O12. 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. Li7-xLa3TaxZr2-xO12 garnets with x = 0.125-2 crystallize in the garnet structure, space group Ia (3) over bard. As the tantalum content increases, the lattice parameter at room temperature decreases from a = 12.9833(1) angstrom for Li6.875La3Ta0.125Zr1.875O12 down to a = 12.81224(7) angstrom for Li5La3Ta2O12. In Li6.5La3Ta0.5Zr1.5O12 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 Li7La3Zr2O12 tetragonally derived garnet and statistically disordered Li6.5La3Ta0.5Zr1.5O12 garnet, lithium partitioning remains unchanged as temperature decreases. (C) 2011 Elsevier B.V. All rights reserved.