J. Emery

A distribution of activation energies for the local and long-range ionic motion is consistent with the disordered structure of the perovskite Li3xLa2/3-xTiO3

Abstract 7 Li nuclear magnetic resonance spin–lattice relaxation time T1 versus temperature is reported in the 150 K–900 K temperature range on lithium lanthanum titanate fast ionic conductors. Because of the presence of disorder in the distribution of the lanthanum ions in the crystalline structure of this oxide and consequently in the conduction pathways of the lithium ions we propose to explain the strong asymmetry shown by these T1 versus 1/T curves by assuming independent ionic hops over a distribution of activation energies for the thermally activated Li+ ion hops. According to this assumed model the spin–lattice relaxation times T1 and the DC conductivity are fitted consistently in the 200–600 K and 300–400 K temperature ranges respectively. For both lower and higher temperatures a departure of the experimental data from the model is observed and explained. The use of this model to fit both T1 and DC conductivity data ruled out the possibility that different forms of the distribution would lead to a reasonable representation of T1. The physical meaning of the obtained parameters is discussed in accordance with the structure of the compounds.

Nuclear magnetic resonance investigation of Li+-ion dynamics in the perovskite fast-ion conductor Li3xLa2/3-x□1/3-2xTiO3

7Li nuclear magnetic resonance relaxation times T1, T1ρ and T2 versus temperature are reported in the 150-900 K temperature range for the lithium lanthanum titanate Li3xLa2/3-x1/3-2xTiO3 perovskite-type fast-ionic conductors. The presence of Li+ ions of two kinds with slightly differing environments is displayed in these experiments. These ions exhibit two different motions: a fast one with a characteristic frequency around 100 MHz at 350 K and a slow one whose frequency is around 60 kHz at 280 K. These two different Li+ species cannot be differentiated by means of the fast motion (only one T1 is observed from the experiments), but only by means of the slow ones (two T1ρ and two T2 are observed). These motions are respectively attributed to Li+ motion inside the A-cage of the perovskite structure formed by the oxygen ions and to Li+ hops between the cages. T1- and T1ρ-studies also performed on the 6Li nucleus clearly show that just dipolar nuclear interaction is responsible for Li+ relaxation. This result is at variance with what has been previously put forward for the relaxation process in these compounds.

Topotactic H+/Li+ ion exchange on La2/3−xLi3xTiO3: new metastable perovskite phases La2/3−xTiO3−3x(OH)3x and La2/3−xTiO3−3x/2 obtained by further dehydration

We have synthesized novel La2/3−xTiO3−3x(OH)3x (0.07 < x < 0.13) compounds by ion exchange of the lithium ions from La2/3−xLi3xTiO3 in dilute HNO3 at 60°C. The proton derivatives crystallize in tetragonal perovskite cells (a ≈ ap, c ≈ 2ap) similar to those of the parent oxides. Interestingly, the protonated oxides (characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and 1H nuclear magnetic resonance (NMR) yield new metastable A-site- and oxygen-deficient perovskites La2/3−xTiO3−3x/2 at about 800°C.

Polaronic effects on lithium motion in intercalated perovskite lithium lanthanum titanate observed by 7Li NMR and impedance spectroscopy

The long-range and short-range motion of lithium ions into an electrochemically intercalated Li3xLa2/3-xTiO3 (LLTO) sintered pellet has been studied by ac impedance spectroscopy and 7Li solid state nuclear magnetic resonance (NMR). The temperature dependence of the dc conductivity and the intercalation ratio dependence of the chemical shift, the relative intensity of the resonance line, the spin-lattice and the spin-spin relaxation times of 7Li NMR experiments are indicative of polaron formation at the initial stage of intercalation. The total dc conductivity measured in the temperature range 45-600 K and the shape of the impedance diagrams show that after intercalation the conductivity is both ionic and electronic in nature. However for temperatures lower than 300 K the total conductivity is mainly dominated by the electronic one and for temperatures higher than 400 K the total conductivity is dominated by the ionic one. Moreover at low temperatures, when electronic conductivity dominates, the temperature dependence of the conductivity agrees well with a polaron model for conduction in these intercalated oxides. The NMR experiments clearly show that the resonance peak decreases strongly as intercalation occurs. This is explained by a coupling between the electronic and the Li+ nuclear spins leading to an unobservability of some Li+ nuclei in NMR. The other Li+ nuclei, which do not interact directly with the electronic spins, are responsible for the observed NMR signal. If the static effect of the intercalation is weak and leads to a very small chemical shift variation, variable temperature 7Li NMR spin-lattice and spin-spin relaxation measurements show that the presence of electrons acts essentially on the dynamics of the Li+ nuclei through the lattice modification induced by the polaron formation. At the beginning of the intercalation the inverse of the relaxation time T1 of the observed Li+ nuclei decreases by one order of magnitude. At the same time the linewidth of the resonance peak (1/T2) decreases abruptly. The motion of the lithium ions is sharply enhanced. For further intercalation, 1/T1 decreases and the linewidth of the central peak increases indicating that the variations of the relaxation times are mostly governed by the variations of the spectral densities of the Li+ motion. Consequently, the lithium motion decreases gradually as intercalation proceeds. These results are in good agreement with a lattice modification due to polaron formation during intercalation.

Broadband dielectric spectroscopy study of Li+ ion motions in the fast ionic conductor Li3xLa2/3−xTiO3 (x = 0.09); comparison with 7Li NMR results

Microscopic motions of Li+ ions in the fast ionic conductor Li3xLa2/3−xTiO3 (x = 0.09) are studied by dielectric spectroscopy in the frequency range from 103 to 4 × 109 Hz and in the temperature range from 200 to 400 K. Several dielectric relaxations are evidenced by this technique and can be ascribed to different motions of the Li+ ions in the oxide. These motions are related to the Li+ motions observed by means of 7Li NMR and dc conductivity and already reported in previous papers. From these two complementary techniques, three motions of Li+ ions are evidenced in the perovskite structure ABO3: a slow motion that corresponds to the hopping of the Li+ ions from one A-cage to the next vacant one through bottlenecks made of four oxygen ions and two fast motions that correspond to local motions of the mobile ions between their off-centred positions in the A-cage of the perovskite structure. A change in the mechanism of conduction is observed around 200 K. This change is attributed to a change in the dimensionality of the Li+ ion motion from 2D to 3D as temperature is increased. At low temperatures (T<200 K) both the local and the long range Li+ ion motions happen in the (a,b) planes of the crystallographic structure (2D motion). As temperature increases, Li+ ions experience the entire volume of the A-cage finally moving in three directions above 400 K (3D motion). This change is corroborated by the ratio of the activation energies in the two domains, i.e. 1.5, observed in T1 versus 103/T plots as well as in the dc conductivity plot and in the dielectric relaxations versus 103/T plot. These results confirm the fact that, in Li3xLa2/3−xTiO3, the long range motion of Li+ ions is evidenced by T1ρ and σdc and their local motions in the A-site of the perovskite structure are evidenced by T1 and by dielectric spectroscopy at frequencies higher than 1 MHz, in the temperature range investigated. Therefore, T1ρ and σdc can be compared since they are related to the same ionic motion. Finally, we found that the constant loss behaviour, observed by previous authors, is in fact the contribution of two quasi-Debye dielectric relaxations.