An Instant Change of Elastic Lattice Strain during Cu 2 Se Phase Transition: Origin of Abnormal Thermoelectric Properties

Abstract The superionic conductor Cu 2 Se is a promising thermoelectric material due to its low thermal conductivity. An abnormal but clear change in the thermoelectric parameters has been observed during the phase transformation from the ordered and non‐cubic α‐Cu 2 Se to the disordered and cubic β‐Cu 2 Se. However, the microstructural origin of the abnormal change and its implications for thermoelectric applications remain largely unknown. Herein, by mimicking the real working conditions of thermoelectrics, the phase transition from α‐ to β‐Cu 2 Se induced by the rising temperature has been carefully investigated by in situ transmission electron microscopy. It is observed that an abrupt and anisotropic volume‐change in the Se‐sublattice occurs when the temperature is raised to the phase transition point. The abnormal change in the crystalline volume versus temperature, which is caused by the local migration of Cu‐ions, induces an instant and uncommon strain‐field, which reduces the carrier's mobility and increases the electrical resistance. Local migration of Cu‐ions is responsible for a quite low thermal conductivity. Such effects exist only at the instance of the phase transition. Observing the thermoelectric response of the structure during the phase transition may provide insights into the development of high performance thermoelectric materials, which fall beyond the traditional approaches.