Fast Na+ Ion Conduction in NASICON-Type Na3.4Sc2(SiO4)0.4(PO4)2.6 Observed by 23Na NMR Relaxometry

The bulk diffusion of Na in Na3.4Sc2(SiO4)0.4(PO4)2.6 was investigated by 23Na NMR relaxometry in the temperature range from 250 to 670 K. These measurements reveal fast Na diffusion with hopping rates of 3 × 108 s–1 for the Na+ ions at 350 K and activation barriers for single Na+ ion jumps of (0.20 ± 0.01) eV. From these values a diffusion coefficient of D = 6.4 × 10–12 m2/s and a Na ion conductivity of σNa = 4 mS/cm (both at 350 K) can be estimated. Measurements on two samples, one stored in air and one stored in Ar, do not show significant differences, which reveals that these NMR measurements are probing the bulk diffusion while conductivity measurements usually are also influenced by grain boundaries that can be affected by the moisture level during storage.