Microstructural control and thermoelectric properties of misfit layered sulfides (LaS)_{1+ m}TS₂ (T = Cr, Nb): The natural superlattice systems

We investigated the high-temperature thermoelectric properties of misfit layered n-type (LaS)_1.20CrS₂ and p-type (LaS) _1.14NbS₂. 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 ∼0.9 and ∼0.5 W K ^-1 m^-1, respectively, at 950 K) perpendicular to the pressing axis. The improved electrical conductivities of the oriented CrS ₂ and NbS₂ samples resulted in high power factors of 170 and 410 μW 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)_1.20CrS₂ system, whereas the weakly/randomly oriented texture produced the highest ZT of 0.15 at 950 K among the (LaS)_1.14NbS₂ system. These misfit layered sulfides exhibit phonon glass-electron crystal behavior and provide tremendous opportunities for further enhancing ZT by optimizing the thermoelectric properties.