Extremely low thermal conductivity and high thermoelectric performance in liquid-like Cu2Se1−xSx polymorphic materials

Abstract

Recently, copper chalcogenides Cu_2−xδ (δ = S, Se, Te) have attracted great attention due to their exceptional thermal and electrical transport properties. Besides these binary Cu_2−xδ compounds, the ternary Cu_2−xδ solid solutions are also expected to possess excellent thermoelectric performance. In this study, we have synthesized a series of Cu₂Se_1−xS_x (x = 0.2, 0.3, 0.5, and 0.7) solid solutions by melting the raw elements followed by spark plasma sintering. The energy dispersive spectroscopy mapping, powder and single-crystal X-ray diffraction and X-ray powder diffraction studies suggest that Cu₂Se and Cu₂S can form a continuous solid solution in the entire composition range. These Cu₂Se_1−xS_x solid solutions are polymorphic materials composed of varied phases with different proportions at room temperature, but single phase materials at elevated temperature. Increasing the sulfur content in Cu₂Se_1−xS_x solid solutions can greatly reduce the carrier concentration, leading to much enhanced electrical resistivity and Seebeck coefficients in the whole temperature range as compared with those in binary Cu₂Se. In particular, introducing sulfur at Se-sites reduces the speed of sound. Combining the strengthened point defect scattering of phonons, extremely low lattice thermal conductivities are obtained in these solid solutions. Finally, a maximum zT value of 1.65 at 950 K is achieved for Cu₂Se_0.8S_0.2, which is greater than those of Cu₂Se and Cu₂S.