Abstract
Thermoelectric (TE) materials can directly switch waste-heat into useful electric energy and evident significant role to future energy management. In recent years, Sb based chalcogenides have been widely studied due to their excellent TE performance. Based on this, the present work focuses on preparation, crystal structure determination and thermoelectric studies of the new Co-Sb-Se ternary system. The obtained needle shaped single crystal specimens of Co-Sb-Se phase are subjected to single crystal diffraction studies, the refinement of crystal data reveals that the material adopts FeAs2 type orthorhombic structure [Marcasite, space group Pnnm (58); Pearson symbol = oP6, a = 4.9864(3) Å b = 5.9657(3) Å c = 3.6933(2) Å]. It is noticed that the pristine Co2.0Sb1.6Se2.4 exhibits merely low thermal conductivity value (∼1.0 Wm−1K−1 at 300 K and ∼0.7 Wm−1K−1 at 657 K) due to severe phonon scattering among Sb/Se disordered lattices, analogously the compound shows poor electrical transport properties i.e. low power factor (σS2). Hence suitable structural modification is carried out to improve power factor values of the compound. Effect of Zn and Sn substitution in the pristine Co2.0Sb1.6Se2.4 system has been potentially analyzed. The qualitative and quantitative structural analyses of the compounds were assessed by Rietveld refinement technique. As expected, upon substitution of Zn and Sn atoms, the σS2 values of the samples considerably increased. Additionally, the compounds exhibit ultra low thermal conductivity ∼0.5 Wm−1K−1 at 657 K via phonon scattering by point defects, which originates reduction in lattice thermal conductivity due to mass and strain fluctuations among the host (Sb/Se disordered site) and guest atoms. Therefore, the low thermal conductivity together with enhanced electrical transport properties result in higher thermoelectric figure of merit (ZT) for Co2Sb1.5Zn0.1Se2.4 and Co2Sb1.5Sn0.1Se2.4 than that of pristine Co2Sb1.6Se2.4 with an optimal carrier concentration. As a result, a peak ZT value of ∼0.5 is found in Co2Sb1.5Zn0.1Se2.4 compound at 657 K which is analogous to the state-of-the-art thermoelectric materials based on ternary chalcogenides.
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