7Li NMR diffusion studies in micrometre-space for perovskite-type Li0.33La0.55TiO3 (LLTO) influenced by grain boundaries

Perovskite-type oxide solid electrolyte Li0.33La0.55TiO3 (LLTO) samples are known to have high ionic conductivities and grain-boundary resistances from AC impedance spectroscopy. In addition, the existence of domain structures has been shown by transmission electron microscopy and other techniques. In this study Li+ migration in tetragonal and cubic samples (t- and c-LLTO) was studied in micrometre space by pulsed-gradient spin-echo (PGSE) NMR from the room temperature up to 100 °C. The grain-boundary effects appeared clearly in the 7Li echo attenuation plots, which were not linear and included at least two components. Previously, we reported that the Li+ diffusion in inorganic solid electrolytes (garnets, NASICON, sulfides) distributes widely in time and space, illustrated by the dependences of the measuring parameters such as observation time (Δ) and pulsed-field gradient (PFG) strength (g). In addition to the complexities of parameter dependent Li+ diffusion, grain-boundary disturbance effects were observed for Li+ diffusion phenomena in LLTO samples. We indicated that a unique DLi value could be estimated from the linear echo attenuation plot at convergent measuring conditions with long Δ and large g for garnets and NASICON. The LLTO samples showed curved echo attenuation plots at the convergent measuring conditions, which were analysed by two components to give two Li+ diffusion constants (DLi-fast and DLi-slow). These values are consistent with the tracer diffusion constants measured at higher temperatures (>150 °C). For c-LLTO, DLi-fast and DLi-slow showed good correspondences with the bulk ionic conductivity (σbulk) and grain-boundary ionic conductivity (σgb), respectively. The Li carrier numbers estimated from DLi-fast and σbulk for c- and t-LLTO showed similar values, whereas those estimated from DLi-slow and σgb for c-LLTO were smaller by approximately one-order of magnitude.