Preparation and Thermoelectric Properties of Misfit-Layered Sulfide [Yb1.90S2]0.62NbS2

We have prepared polycrystalline samples of Yb-Nb-S-based misfit-layered compound and measured their thermoelectric properties below 300 K. The compound consists of an alternate stacking of a trigonal prism (MoS2)-type [NbS2] layer and a two-layered rock-salt-type [Yb1.90S2] slab, being incommensurate parallel to the a-axis. Approximately 5% Yb deficiency is necessary to obtain single-phase samples. Metallic conductivity accompanied by a large Seebeck coefficient S has been observed down to 4.2 K for the single-phase samples. At 300 K, the sample exhibits S = 60 µV·K-1 and resistivity of ρ= 1.4 mΩ·cm, with thermal conductivity of κ= 0.80 W·K-1·m-1. The resulting figure-of-merit (Z = S 2/ρκ) becomes 3.2×10-4 K-1 at 300 K, yielding the dimensionless figure-of-merit of Z T∼0.1.