Enhancing the Thermoelectric Properties of Misfit Layered Sulfides (MS)1.2+q(NbS2)n (M = Gd and Dy) through Structural Evolution and Compositional Tuning

The misfit monolayered sulfides, (GdS) 1.20 NbS 2 , (DyS) 1.22 NbS 2 , (Gd 0.1 Dy 0.9 S) 1.21 NbS 2 , (Gd 0.2 Dy 0.8 S) 1.21 NbS 2 , and (Gd 0.5 Dy 0.5 S) 1.21 NbS 2 and the misfit bilayered sulfide (GdS) 0.60 NbS 2 were synthesized via sulfurization under flowing CS 2 /H 2 S gas and consolidated by pressure-assisted sintering. The thermoelectric properties of the monolayered and bilayered sulfides perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction were investigated over a temperature range of 300-873 K. The crystal grains in all the sintered samples were preferentially oriented perpendicular to the pressing direction, which resulted in highly anisotropic electrical and thermal transport properties. All the sintered samples exhibited degenerate n -type semiconductor-like behavior, leading to a large thermoelectric power factor. The misfit layered structure yielded low lattice thermal conductivity. The evolution of the monolayered structures into bilayered structures affected their thermoelectric properties. The thermoelectric figure of merit ( ZT ) of monolayered (GdS) 1.20 NbS 2 was higher than that of bilayered (GdS) 0.60 NbS 2 due to the larger power factor and lower lattice thermal conductivity of (GdS) 1.20 NbS 2 . The lattice thermal conductivity of the monolayered sulfide was lower in (Gd x Dy 1- x S) 1.2+ q NbS 2 solid solutions. The large power factor and low lattice thermal conductivity allowed a ZT value of 0.13 at 873 K in (Gd 0.5 Dy 0.5 S) 1.21 NbS 2 perpendicular to the pressing direction.