Synthesis and crystallographic studies of garnet-related lithium-ion conductors Li6CaLa2Ta2O12 and Li6BaLa2Ta2O12

High-purity specimens of Li6CaLa2Ta2O12 and Li6BaLa2Ta2O12 have been successfully synthesized by solid-state reactions. The analytical chemical compositions of these samples were in good agreement with the nominal compositions of Li6CaLa2Ta2O12 and Li6BaLa2Ta2O12. The Rietveld refinements verified that these compounds have the garnet-type framework structure with the lattice constants of a = 12.725(2) Å for Li6CaLa2Ta2O12 and a = 13.001(4) Å for Li6BaLa2Ta2O12. All of the diffraction peaks of X-ray powder diffraction patterns were well indexed on the basis of cubic symmetry with space group Ia-3d. To make a search for Li sites, the electron density distributions were precisely examined by using the maximum entropy method. Li+ ions occupy partially two types of crystallographic site in these compounds: (i) tetrahedral 24d sites, and (ii) distorted octahedral 96h sites, the latter of which are the vacant sites of the ideal garnet-type structure. The present Li6CaLa2Ta2O12 and Li6BaLa2Ta2O12 samples exhibit the conductivity σ = 2.2 × 10− 6 S cm− 1 at 27 °C (Ea = 0.50 eV) and σ = 1.3 × 10− 5 S cm− 1 at 25 °C (Ea = 0.44 eV), respectively.