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 x 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