Ionic conductivity of Dion-Jacobson type oxide LiLaTa2O7 and oxynitride LiLaTa2O6.15N0.57 measured by impedance spectroscopy

Ionic transport in the layered perovskite, LiLaTa2O7, and its oxynitride derivative, LiLaTa2O6.15N0.57, were examined by ac impedance spectroscopy in a N2 atmosphere. Non-Arrhenius behavior was observed when LiLaTa2O7 was heated from 30 °C to 70 °C with the conductivity decreasing from 10−5.9 S/cm to 10−6.3 S/cm, due to the heat-induced removal of interlayer water. The conductivity of dehydrated LiLaTa2O7 then increased monotonously from 10−6.3 S/cm at 80 °C to 10−3.7 S/cm at 283 °C with an activation energy (Ea) of 0.54 eV. Compared to LiLaTa2O7, the oxynitride phase, LiLaTa2O6.15N0.57, exhibited much lower conductivity ranging from 10−6.7 S/cm at 315 °C to 10−5.5 S/cm at 496 °C with an Ea of 0.57 eV. The distinct ionic conductivity among the similar layered perovskites, LiLaTa2O7, LiLaTa2O6.15N0.57, Li2La2/3Ta2O7, and Li2LaTa2O6N, highlights the importance of structural factors on the ionic conductivity, particularly, the site occupancy of the mobile cation, Li+, and the connectivity of the anion sublattice.