Dependence of the Li-Ion Conductivity and Activation Energies on the Crystal Structure and Ionic Radii in Li₆MLa₂Ta₂O₁₂

Abstract

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(2+) 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 M(2+). As may be expected, the unit-cell parameters increase linearly with increasing ionic radius from Ca(2+) over Sr(2+) to Ba(2+), 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.

Gallium-Doped Li7La3Zr2O12 Garnet-Type Electrolytes with High Lithium-Ion Conductivity

“…Generally, the direct lithium-ion transport along tetrahedral sites is difficult; Li NMR and dielectric loss spectroscopy measurements suggest a pathway of 48g/96h-48g/96h involving only octahedral sites 57,[59][60][61] . However, anisotropic atomic displacement parameters obtained using neutron diffraction, maximum entropy methods and two-dimensional 6 Li- 6 Li exchange NMR spectra, suggest that the diffusion occurs through the pathway 24d-48g/96h-24d 29, 62,63 . As the jumping rate of lithium at tetrahedral sites is lower than that at octahedral sites, the lithium-ion migration rate through the 24d-96h-24d pathway is limited by the jumping rate of lithium at tetrahedral sites.…”

Structural limitations for optimizing garnet-type solid electrolytes: a perspective

“…20 The increasing ionic radius from Ca 2+ over Sr 2+ to Ba 2+ increases the polyhedron volume of the La position, the site which is also occupied by the alkaline earth cation. While these polyhedra expand (see connectivity representation in Fig.…”

“…This induces an expansion of the Li-O polyhedra due to chemical pressure on the easily polarizable Li-O bond, ultimately leading to broader diffusion pathways and higher ionic conductivities. 20 …”

“…20 However, the knowledge of the underlying structural mechanism also demonstrates the limitation of this possibility. On the one hand, a substitution on the Li sites will result in a lower Licontent and a possible disruption of the diffusion pathways, while the substitution on the octahedral MO 6 sites might not result in much higher mobilities.…”

Li6Sr(La1−xEux)2Sb2O12(0<x≤1.0) solid-solution red phosphors for white light-emitting diodes

“…The refinement is proceeded by used the structural data for Li 5 La 3 Sb 2 O 12 given by Cussen and Yip [17] as an initial model, which is applied to assume the Sr substitutes La on its crystallographic site [18]. The position of Sr 2 þ and La 3 þ cations are constrained on Wyckoff position 24c, Sb 3 þ cations are on Wyckoff position 16a, Li þ cations are distributed over Wyckoff position 24d and the interstitial pseudo-octahedral 48 g [19,20]. As expected, the crystallographic structure of Li 6 SrLaEuSb 2 O 12 is a cubic system with the lattice parameters of a=b=c=12.77 Å, which are slightly smaller than those of the pure Li 6 SrLa 2 Sb 2 O 12 crystal (a=b=c=12.893 Å) reported previously [18].…”