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

Recently, copper chalcogenides Cu2-x delta (delta = S, Se, Te) have attracted great attention due to their exceptional thermal and electrical transport properties. Besides these binary Cu2-x delta compounds, the ternary Cu2-x delta solid solutions are also expected to possess excellent thermoelectric performance. In this study, we have synthesized a series of Cu2Se1-xSx (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 Cu2Se1-xSx 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 Cu2Se1-xSx 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 Cu2Se0.8S0.2, which is greater than those of Cu2Se and Cu2S.