Thermoelectric properties of sintered polycrystalline ZnIn₂S₄

Ceramic compacts of spinel-type ZnIn₂S₄ and IIIa-ZnIn₂S₄ polytype with a layer structure were synthesized by the reaction-sintering of mixed powders of ZnS and In₂S₃ at 723 K and 1073 K in Ar (containing 1° H₂) atmosphere, respectively. The thermoelectric properties were investigated in the temperature range from 473 to 873 K. Thermoelectric figure of merit of the IIIa type was much larger than that of the spinel type, and it was slightly higher than the figure of merit of (ZnO)₉In₂O₃, which is known to show the largest value among the oxide homologous compounds. To improve the thermoelectric properties, a c-plane-oriented sintered body of the IIIa polytype was successfully fabricated by a usual ceramic process. The figure of merit in the direction on the c plane was larger than on the ab plane due to higher electrical conductivity on the c plane and increased with increasing temperature showing the largest value of 1.3 × 10⁻⁴ K⁻¹ at 873 K.