Thermoelectric properties of Co substituted synthetic tetrahedrite

Graphical abstract: Tetrahedrite compounds Cu_12−xCo_xSb₄S₁₃ (0 ⩽ x ⩽ 2.0) were prepared by solid state synthesis. XPS revealed that Co partially substitutes Cu¹⁺ as well as Cu²⁺. A systematic trend in the increase of resistivity and Seebeck coefficient with rising of doping content was observed. Also, a systematic decrease of total thermal conductivity with doping was found. As a combined effect of power factor and total thermal conductivity, the maximum thermoelectric figure of merit (zT) 0.98 was obtained at 673 K for Cu_11.5Co_0.5Sb₄S₁₃. Display Omitted - Abstract: Transition metal atom (Co) substituted synthetic tetrahedrite compounds Cu_12−xCo_xSb₄S₁₃ (x = 0, 0.5, 1.0, 1.5, 2.0) were prepared by solid state synthesis. X-Ray Diffraction (XRD) patterns revealed tetrahedrite as the main phase, whereas for the compounds with x = 0, 0.5 a trace of impurity phase Cu₃SbS₄ was observed. The surface morphology showed a large grain size with low porosity, which indicated appropriate compaction for the hot pressed samples. The phase purity, as monitored by Electron Probe Micro Analysis (EPMA) is in good agreement with the XRD data. The elemental composition for all the compounds almost matched with the nominal composition. The X-ray Photoelectron Spectroscopy (XPS) data showed that Cu existed in both +1 and +2 states, while Sb exhibited +3 oxidation states. Elastic modulus and hardness showed a systematic variation with increasing Co content. The electrical resistivity and Seebeck coefficient increased with increase in the doping content due to the decrease in the number of carriers caused by the substitution of Co²⁺ on the Cu¹⁺ site. The positive Seebeck coefficient for all samples indicates that the dominant carriers are holes. A combined effect of resistivity and Seebeck coefficient leads to the maximum power factor of 1.76 mW m⁻¹ K⁻² at 673 K for Cu_11.5Co_0.5Sb₄S₁₃. This could be due to the optimization in the carrier concentration by the partial substitution of Co²⁺ on both the Cu¹⁺ as well as Cu²⁺ site at the same doping levels, which is also supported by the XPS data. The total thermal conductivity systematically decreased with increase of doping content as it is mainly influenced by the decrease of carrier thermal conductivity. The maximum thermoelectric figure of merit zT = 0.98 was obtained at 673 K for Cu_11.5Co_0.5Sb₄S₁₃.