Nanostructural and Microstructural Ordering and Thermoelectric Property Tuning in Misfit Layered Sulfide [(LaS)x]1.14NbS2

The potential to control the structural ordering at the nanoscale in misfit layered systems can open new paths for achieving high thermoelectric performance in these systems. In the present work, we demonstrate that compositional changes can provide nanoscale tuning in misfit layered (LaxSx)(1.14)NbS2 (x = 0.90, 0.95, 1.00, 1.05, 1.10), leading to improved thermoelectric properties (investigated over the temperature range of 300 to 950 K). The samples were prepared by CS2 sulfurization and consolidated using pressure-assisted sintering. It was revealed through transmission electron microscopy analysis that non-stoichiometry promotes long-range ordering of the layers and results in elongated lamella formations. The La deficient x = 0.95 sample was found to contain large strain induced stacking disorder, owing to which the long lamellas rolled up to form tubular structures, whereas the La rich x = 1.05 sample had a much ordered structure and only slight curling of lamella edges was observed. Improved structural ordering and textured grain growth in La rich sample resulted in a similar to 30% improved power factor (similar to 460 mu W K-2 m(-1) at 950 K) and ZT (similar to 0.2 at 950 K) in the in-plane direction, compared to the x = 1.00 sample. The origin of very low thermal conductivity (1.1-2.5 W K-1 at 950 K) in our samples has been identified and discussed in detail.