CsCuS: Promising Thermoelectric Material with Enhanced Phase Transition Temperature.

CuS, featuring low cost, nontoxicity, and earth abundance, has been recently recognized as a high efficiency thermoelectric (TE) material. However, before reaching the maximum of the figure of merit ( ZT), CuS undergoes three phase transformations starting at 370 K, which give rise to severe problems, such as possible decomposition and low reliability. Herein, we discover CsCuS with phase transformation at 823 K, which is significantly higher than the 370 K value of CuS. Single crystal diffraction data reveal that its two phases are constructed by the same CuS columnar building unit via propagating either at the opposite sides into a layered o-CsCuS, or at the four apexes into a 3D t-CsCuS, respectively. Interestingly, the o-to- t transformation is quick, but the reverse one is relatively slow. Theoretical studies reveal that the CuS column exhibits not only the most condensed atomic aggregation ( D) but also the lightest effective mass ( m*), along which higher σ is realized. More interestingly, both phases exhibit remarkable ZT enhancements, 0.46 at 800 K for o-CsCuS, and 0.56 at 875 K for t-CsCuS, which are 170% and 175% that of CuS at the same temperature.