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

Structural properties of Li7 − xLa3TaxZr2 − xO12 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 Li7La3Zr2O12 was solved. It was shown that the tetragonally derived garnet structure of space group I41/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¯d. As the tantalum content increases, the lattice parameter at room temperature decreases from a = 12.9833(1) Å for Li6.875La3Ta0.125Zr1.875O12 down to a = 12.81224(7) Å 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.

► Structural properties of Li7–xLa3TaxZr2–xO12 garnets for x=0–2.
► Lithium ion conductor as solid state electrolyte.
► 0 < x < 0.125, a transition occurs from ordered to disordered lithium configuration.
► Tetragonal garnet is the thermodynamically stable structure for Li7La3Zr2O12.