Anomalies in Li + ion dynamics observed by impedance spectroscopy and 7Li NMR in the perovskite fast ion conductor (Li 3 xLa 2/3- x□ 1/3-2 x)TiO 3

Abstract

A previous study, performed on the fast ionic conductor Li 3 x La 2/3- x TiO 3 by 7Li and 6Li Nuclear Magnetic Resonance (NMR), has shown that Li + ions undergo two different motions: a fast motion ( τ c≈10 −9 s) inside the A-cage of the perovskite structure and a slower one ( τ c≈10 −6 s) from one A-cage to a next vacant one. Furthermore, a change of these two motion mechanisms is observed around 200 K. Apart from NMR, impedance spectroscopy may also afford information on the ionic motion mechanism. Lithium motion in Li 3 x La 2/3- x TiO 3 is then studied by impedance spectroscopy in the 1 Hz–10 MHz frequency range and in the 140–500 K temperature range. The results obtained by these two techniques, i.e. 7Li NMR and impedance spectroscopy, are then compared in the 140–270 K temperature range. As observed in NMR, the dc conductivity shows a change in the mechanism of ionic motion around 200 K. Apart from the dc plateau, the real part of conductivity ( σ′) displays a dispersive behavior at high frequencies. Plotting the ac data in terms of impedance and modulus reveals the presence, in the mechanism of conduction, of both a nonlocalized conduction (long-range motion of the mobile ions) and a localized one (dipolar relaxation). According to these experimental observations, an equivalent electrical circuit is proposed, taking into account the physical processes assumed to be present when a small electrical signal is applied to the oxide. Both dipole polarization and long-range motion of the mobile ions are included in the electrical circuit of the conductive pathways. A complex nonlinear least squares fitting procedure (CNLS) is used to fit this electrical model to the experimental conductivity vs. frequency response ( σ′ and σ″). This procedure shows that all the parameters linked to the conductive pathways undergo a sudden change around 200 K, suggesting that a change in the ionic motion mechanism occurs at this temperature. This result is discussed in relation to both the crystallographic structure of the ionic conductor and the results previously obtained by 7Li NMR.