Li+ ionic conduction in the layered perovskite Li2La2/3Ta2O7

Abstract

The Li⁺ ionic conduction properties of the Li₂La_2/3Ta₂O₇ layered perovskite compound have been investigated by complementary techniques: impedance spectroscopy, ⁷Li NMR and thermal neutron powder diffraction. Up to 770 K, the results are consistent with an electrical conductivity dominated by Li⁺ ions jumping between Li1 and Li2 sites, the adjacent centers of the two kinds of LiO₄ tetrahedra constituting the interlayer region of the structure. The sudden event, observed near 770 K on the curve log(σT) = f(1000/T), is associated with significant changes to the ⁷Li NMR signal and to a structural modification followed by neutron powder diffraction up to 973 K. All these facts are consistent with the displacement at 770 K of the lithium ions residing in the Li1 sites. Neutron powder diffraction showed that, in the interlayer region, the location of the lithium ions in the Li2 sites remains unchanged. However, 17% of the Li1 population leaves its tetrahedral position to occupy a new Li3 site inside the perovskite cages, very close to their four O²⁻ bottlenecks. The other Li⁺ ions (83%) remain in the interlayer but their coordination changes from tetrahedral to a five fold one.