Dependence of the li-ion conductivity and activation energies on the crystal structure and ionic radii in Li₆MLa₂Ta ₂O₁₂

Inspired by the promising ionic conductivities of the lithium conducting garnets, we present a comparative study on the influence of the ionic radius of M²⁺ on the 8-coordinate site and the crystal structure on the ionic transport in the solid solution Li₆MLa₂Ta ₂O₁₂. Neutron diffraction and synchrotron diffraction in combination with AC impedance measurements are employed to understand the systematic substitution with different-sized alkaline earth cations M ²⁺. As may be expected, the unit-cell parameters increase linearly with increasing ionic radius from Ca²⁺ over Sr²⁺ to Ba²⁺, accompanied by an increase in the polyhedral volumes of the dodecahedral, and tetrahedral positions and the ionic conductivities. While the TaO₆ octahedral volume remain constant, the anisotropic thermal parameters of the coordinating oxygen anions suggest a high degree of rotational freedom with increasing unit-cell size. These structural parameters lead to lower activation energies because of broader Li conduction pathways and a higher flexibility in the crystal lattice, ultimately controlling the ionic conductivities in this class of materials.