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Dependence of the Li-Ion Conductivity and Activation Energies on the Crystal Structure and Ionic Radii in Li<sub>6</sub>MLa<sub>2</sub>Ta<sub>2</sub>O<sub>12</sub>
Inspired
by the promising ionic conductivities of the lithium conducting garnets,
we present a comparative study on the influence of the ionic radius
of M<sup>2+</sup> on the 8-coordinate site and the crystal structure
on the ionic transport in the solid solution Li<sub>6</sub>MLa<sub>2</sub>Ta<sub>2</sub>O<sub>12</sub>. 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<sup>2+</sup>. As may be expected, the unit-cell parameters
increase linearly with increasing ionic radius from Ca<sup>2+</sup> over Sr<sup>2+</sup> to Ba<sup>2+</sup>, accompanied by an increase
in the polyhedral volumes of the dodecahedral, and tetrahedral positions
and the ionic conductivities. While the TaO<sub>6</sub> 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