Li Mobility in the Orthorhombic Li_0.18La_0.61TiO₃ Perovskite Studied by NMR and Impedance Spectroscopies

Abstract

Electrical conductivity and NMR relaxation times (T 1 and T 2 ) have been determined in the Li 0.18 La 0.61 TiO 3 perovskite. At room temperature, the unit cell constants are a ) 3.865-(1), b ) 3.876(1), and c ) 7.788(2) Å and the space group Pmmm (orthorhombic). In this doubled perovskite, the Rietveld analysis of the X-ray powder pattern showed that La ions occupy preferentially one type of sites (z/c ) 0), and Li and vacancies accommodate with the remaining La at the second site (z/c ) 0.5). From this fact, Li motion should be favored in the plane ab; however, exchanges of Li between contiguous layers are detected above 200 K by NMR spectroscopy. From T 1 and T 2 NMR data, two main relaxation mechanisms have been detected, which have been ascribed to localized exchanges (200-273 K) and extended motions of Li (above 273 K). The dc conductivity shows a non-Arrhenius temperature dependence, and local activation energies of 0.41 and 0.26 eV were obtained in the low-and high-temperature ranges, respectively. Both NMR and electrical conductivity relaxations are described by "stretched exponential" functions, characteristic of correlated ion motions.

Lithium dynamics in molybdenum disulfide intercalation compounds studied by nuclear magnetic resonance

“…The experimental 7 Li NMR spectra in Fig. 1 shows a central line superimposed on a broad base line, which is attributed to a distribution of electric field gradients at the lithium sites (i.e., a distribution of the quadrupolar coupling which smear out the quadrupole satellite structure) [4,11]. The non-decoupled spectrum in Fig.…”

“…Fast exchange process was also detected by 7 Li T 1 measurements in a solid tetrameric form of tert-butyllithium, where the lithium atoms exchange between the four apical sites of a tetrahedron with an exchange rate of 135 MHz at 300 K [24]. Localized exchange of Li between contiguous sites of the perovskite structure was also detected by 7 Li relaxation in the fast ionic conductor Li 0.18 La 0.61 TiO 3 between 220 and 273 K [4].…”

Temperature dependence of the ionic conductivity in Li3xLa2/3−xTiO3: Arrhenius versus non-Arrhenius

“…Further experimental details on sample preparation have been published elsewhere. 20,22 The experimental setup to measure the electric modulus in the time domain has been described in detail by Richert et al 28 -31 The step in D can be achieved experimentally with a step in charge in the electrodes of the capacitor ͑formed by the solid sample and contacts͒, implemented connecting a voltage supply U 0 for a short time ͑20 s͒ and then isolating the circuit for tϾ0. A value of 4 V has been used here as the initial voltage (U 0 ) to set the charge.…”

“…2͒ as obtained in the frequency domain by using a conventional impedance bridge. 20,22 The relaxation or time decay function of the electric field is reflected in the shape of the electric modulus spectra. The imaginary part M Љ( ) shows a peak with a maximum at a characteristic frequency which is inversely proportional to the temperature dependent relaxation time of the electric-field decay function.…”

“…Previous analysis of these data at temperatures above 180 K show a non-Arrhenius behavior of the conductivity, and a VFT law was proposed to describe the temperature dependence. 20,22 However, our measurements in the time domain allow us to extend the temperature range down to 120 K and the dc conductivity clearly shows an Arrhenius behavior below room temperature down to the lowest temperature, with an activation energy E ϭ0.36Ϯ0.02 eV. The non-Arrhenius temperature dependence of the dc conductivity is then restricted to temperatures approximately above 300 K. 18 -23 This behavior is consistent with the observed saturation of dc conductivity values at high temperatures in fast ionic conductors.…”