Synthesis of multinary rare-earth sulfides PrGdS3, NdGdS3, and SmEuGdS4, and investigation of their thermoelectric properties

We have developed a new strategy for the synthesis of the ternary rare-earth sulfides LnGdS3(Ln: Pr, Nd) and the quaternary rare-earth sulfide SmEuGdS4. This strategy involves the use of a polymerized complex method and sulfurization. Multinary rare-earth oxycarbonate powders were first prepared from nitrates by the polymerized complex method. The oxycarbonates were then sulfurized with CS2 gas to synthesize the multinary rare-earth sulfides. While the sulfurized LnGdS3 powders were crystallized in the orthorhombic α-phase, the sulfurized SmEuGdS4 powder was crystallized in the cubic γ-phase. The α-LnGdS3 transformed completely into γ-LnGdS3 after heat treatment at 1723 K. The γ-LnGdS3 and γ-SmEuGdS4 powders were consolidated by pressure-assisted sintering to fabricate full-density compacts. The Seebeck coefficient, electrical resistivity, and thermal conductivity of the sintered compacts were measured between 300 and 950 K. While the NdGdS3 sintered compact showed an n-type Seebeck coefficient and low electrical resistivity, the SmEuGdS4 sintered compact showed a p-type Seebeck coefficient and high electrical resistivity. The thermal conductivity of NdGdS3 ranged from 0.8 to 1.2 W/K m. The highest figure of merit (ZT=0.29ZT=0.29) was obtained for NdGdS3 at 950 K.