A previous study of the fast ionic conductor Li3xLa2/3-xTiO3 by 7Li and 6Li nuclear magnetic resonance, had shown that Li+ ions undergo 2 different motions: a fast motion (τc ≈ 10-9s) inside the A-cage of the perovskite structure and a slower one (τc ≈ 10-6s) from one A-cage to an adjacent vacant one. Furthermore, a change of these two motion mechanisms was observed around 200K. Apart from nuclear magnetic resonance, impedance spectroscopy may also afford information on the ionic motion mechanism. Lithium motion in Li3xLa2/3-xTiO3 was then studied by impedance spectroscopy in the 1Hz–10MHz frequency range and at 140 to 500K. The results obtained by these two techniques, i.e. 7Li nuclear magnetic resonance and impedance spectroscopy, were then compared for 140 to 270K. As observed in nuclear magnetic resonance, the direct-current conductivity showed a change in the mechanism of ionic motion around 200K. Apart from the direct-current plateau, the real part of conductivity (σ’) displayed a dispersive behavior at high frequencies. Plotting the alternating-current data in terms of impedance and modulus reveals the presence, in the mechanism of conduction, of both a non-localized conduction (long-range motion of the mobile ions) and a localized one (dipolar relaxation). According to these experimental observations, an equivalent electrical circuit was proposed, taking into account the physical processes assumed to be present when a small electrical signal was applied to the oxide. Both dipole polarization and long-range motion of the mobile ions were included in the electrical circuit of the conductive pathways. A complex nonlinear least squares fitting procedure was used to fit this electrical model to the experimental conductivity vs. frequency response (σ’ and σ”). This procedure showed that all the parameters linked to the conductive pathways undergo a sudden change around 200K, suggesting that a change in the ionic motion mechanism occurred at this temperature. This result was considered in relation to both the crystallographic structure of the ionic conductor and the results previously obtained by 7Li NMR.

Anomalies in Li+ Ion Dynamics Observed by Impedance Spectroscopy and 7Li NMR in the Perovskite Fast Ion Conductor (Li3xLa2/3-x1/3-2x)TiO3. O.Bohnke, J.Emery, J.L.Fourquet: Solid State Ionics, 2003, 158[1-2], 119-32