- Observation of the Continuous Phase Transition in Determining the High Thermoelectric Performance of Polycrystalline SnSe.

We report a comprehensive - phase-change study on polycrystalline SnSe via high-temperature X-ray diffraction and - high-voltage transmission electron microscopy from room temperature to 843 K. The results clearly demonstrate a continuous phase transition from to starting from 573 to 843 K, rather than a sudden transition at 800 K. We also find that the thermal-conductivity rise at high temperature after the phase transition, as commonly seen in pristine SnSe, does not occur in SnSe, leading to a high thermoelectric figure of merit. Density functional theory calculations reveal the origin to be the suppression of bipolar thermal conduction in the phase of SnSe due to the enlarged bandgap. This work fills the gap of - characterization on polycrystalline SnSe and provides new insights into the outstanding thermoelectric performance of polycrystalline SnSe.