An Instant Change of Elastic Lattice Strain during Cu₂Se Phase Transition: Origin of Abnormal Thermoelectric Properties

Abstract

The superionic conductor Cu2Se 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‐Cu2Se to the disordered and cubic β‐Cu2Se. 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‐Cu2Se 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.

Recent advances in designing thermoelectric materials

“…335,336 The temperature controlled phase transition in Cu 2 Se has shown another approach for inducing lattice strain and tuning thermoelectric behavior. 337 Crystal geometry is one of the most robust tunable parameters for modulating thermoelectric transport. Structural transformation by the application of temperature and pressure provides an opportunity to explore the large search space without any complicated engineering of the structures.…”

“…335,336 The temperature controlled phase transition in Cu 2 Se has shown another approach for inducing lattice strain and tuning thermoelectric behavior. 337…”

Structural Phase Transition and Related Thermoelectric Properties in Sn Doped AgBiSe2

“…Accordingly, semiconductors with low lattice thermal conductivities have been widely studied and show excellent thermoelectric performance. These semiconductors include Zintl phases [4,5], fast ion conductors [6,7], complex oxides [8,9], and chalcogenides [10].…”

Electrically Tunable Antiferroelectric to Paraelectric Switching in a Semiconductor

“…Remarkably, as documented in Figure 3b and d, the phase boundary is moving quickly and smoothly in the transition, and the sample under observation is quite stable, unlike in the case of a thermally induced phase transition reported in our previous work. 38 This implies that the phase transition is driven mainly electrically. In addition, the temperature measured during the experiment reached only tens of degrees Celsius (Figure S9), far below the phase transition temperature (∼400 K).…”