About the Impact of Defect Phases on the Thermoelectric Properties of Cr₃S_4–xSe_x

Phase relations in Cr₃S₄ and the substituted system Cr₃S_4–x Se _x are studied to determine the influence of chemical substitutions on the thermoelectric properties. In addition to the expected equilibrium phase crystallizing in the monoclinic space group I2/m, some samples exhibit a defect phase with Cr₂S₃‐like structure. The defect phase can be observed in a few samples prior to sintering, with the majority being phase‐pure Cr₃S₄. The defect phase can, however, be introduced in phase‐pure samples through in situ heating. It can be proven that the defect phase has an influence on the thermoelectric properties, by lowering the electrical and thermal conductivity, while increasing the Seebeck coefficient. Substitution in the anion lattice of Cr₃S₄ with Se lowers the thermal conductivity. The improvement is mainly achieved through a reduction of the electronic contribution to the thermal conductivity, leading to total values as low as 1.6 Wm⁻¹ K⁻¹ for the substituted system in comparison to the pristine material 2.3 Wm⁻¹ K⁻¹.