Multiple phase transitions and structural oscillations in thermoelectric Cu2S at elevating temperatures

Abstract The intrinsic ultra-low lattice thermal conductivity (κL) of high performance thermoelectric Cu2S may arise from its multiform complex crystal structures while the further sharp drop of κL observed at elevating temperatures close to the phase transition to hexagonal high-chalcocite is of considerable interest. In this work, in-depth in-situ transmission electron microscopy investigations have been performed by either heating or electron irradiation to analyze the structural evolution of Cu2S. Multiple including new phase structures have been observed in Cu2S. Upon increasing the temperature, drastic structural changes took place including phase transition among different phases, formation of an intermediate phase, switching of crystal orientation and inverse phase transitions, which indicate multiple forms of Cu ordering with high temperature-sensitivity. Such tremendous structural evolution along with formation and movement of phase boundaries are believed to strongly scatter the phonons.