We report a comprehensive in-situ phase-change study on polycrystalline Sn0.98Se via high-temperature X-ray diffraction and in-situ high-voltage transmission electron microscopy from room temperature to 843 K. The results clearly demonstrate a continuous phase transition from Pnma to Cmcm 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 Sn0.98Se, 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 Cmcm phase of Sn0.98Se due to the enlarged bandgap. This work fills the gap of in-situ characterization on polycrystalline Sn0.98Se and provides new insights into the outstanding thermoelectric performance of polycrystalline Sn0.98Se.

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