Switching on fast lithium ion conductivity in garnets: The structure and transport properties of Li_3+xNd₃Te_2-xSb _xO₁₂

Polycrystalline samples of the garnets Li_3+xNd ₃Te_2-xSb_xO₁₂ have been prepared by high temperature solid state synthesis. X-ray and neutron powder diffraction data show that all compounds crystallize in the space group Ia3̄d with lattice parameters in the range 12.55576(12) Å for x = 0.05 to 12.6253(2) Å for x = 1.5. The lithium is distributed over a mixture of oxide tetrahedra and heavily distorted octahedra. Increasing the lithium content in these compounds leads to the introduction of vacancies onto the tetrahedral position and an increasing concentration of lithium found in the octahedra. The latter exhibit considerable positional disorder with two lithium cations positions within each octahedron. Impedance measurements show fast ion conduction with an activation energy of ca. 0.59(6) eV that is largely invariant with composition. Solid-state Li NMR measurements indicate that there is no exchange of lithium between the different coordination environments. These results indicate that lithium conduction in the garnet structure occurs exclusively via a network of edge-linked distorted oxide octahedra and that the tetrahedrally coordination lithium plays no part in the transport properties.