Dependence of the Li-Ion Conductivity and Activation Energies on the Crystal Structure and Ionic Radii in Li6MLa2Ta2O12
2014
Zeier, Wolfgang G. | Zhou, Shiliang | Lopez-Bermudez, Beatriz | Page, Katharine | Melot, Brent C.
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 Li₆MLa₂Ta₂O₁₂. Neutron diffraction and synchrotron diffraction in combination with AC impedance measurements are employed to understand the systematic substitution with different-sized alkaline earth cations M²⁺. 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.
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