Microstructural Control and Thermoelectric Properties of Misfit Layered Sulfides (LaS)1+mTS2 (T = Cr, Nb): The Natural Superlattice Systems

We investigated the high-temperature thermoelectric properties of misfit layered n-type (LaS)(1.20)CrS2 and p-type (LaS)(1.14)NbS2. The samples were prepared by CS2 sulfurization of 6 or 12 h duration and then consolidated using pressure-assisted sintering to produce randomly and highly oriented samples whose microstructures were tunable. Transmission electron microscopy analysis showed that perfectly layered structures containing some stacking faults had formed. The randomly and highly oriented natural superlattices provided ultralow lattice thermal conductivities (as low as similar to 0.9 and similar to 0.5 W K-1 m(-1), respectively, at 950 K) perpendicular to the pressing axis. The improved electrical conductivities of the oriented CrS2 and NbS2 samples resulted in high power factors of 170 and 410 mu W K-2 m(-1), respectively. The highly oriented texture produced the highest thermoelectric figure of merit ZT of 0.14 at 950 K among the (LaS)(1.20)CrS2 system, whereas the wealdy/randomly oriented texture produced the highest ZT of 0.15 at 950 K among the (LaS)(1.14)NbS2 system. These misfit layered sulfides exhibit phonon glass-electron crystal behavior and provide tremendous opportunities for further enhancing ZT by optimizing the thermoelectric properties.