High thermoelectric performance and low thermal conductivity in Cu2-yS1/3Se1/3Te1/3 liquid-like materials with nanoscale mosaic structures

Mosaic-crystal microstructure is one of the optimal strategies for decoupling and balancing thermal and electrical transport properties in thermoelectric materials. Herein, we successfully achieve the desired nanoscale mosaic structures in triple-component Cu2-yS1/3Se1/3Te1/3 solid solutions using Cu2S, Cu2Se, and Cu2Te matrix compounds. They are solved in hexagonal structures with space group R (3) over barm by means of single crystal structural solution and Rietveld refinement. Electron backscatter diffraction measurements show that all the samples are polycrystalline compounds with the grain size in the range of micrometers. However, transmission electron microscopic study reveals that these microscale grains are quasi-single crystals consist of a variety of 10-30 nm mosaic grains. Each mosaic grain is a perfect crystal but titled or rotated with respect to others by a very small angle. In this case, excellent electrical transports are maintained but exceptional low thermal conductivity is achieved throughout the whole temperature range, which is attributed to the combined phonon scatterings by point defects, liquid-like copper ions, and lattice strains or interfaces of mosaic nanograins. Combining all these favorable factors, remarkably high thermoelectric performance is achieved in Cu1.98S1/3Se1/3Te1/3 with a maximum zT of 1.9 at 1000 K.