Structural Evolution and Polymorph-Dependent Thermoelectric Properties of In2Se3

Van der Waals semiconductor In2Se3 exhibits multiple polymorphs, yet the thermal-history-dependent selection of bulk polymorphs and its relationship with transport remain unclear. Here, phase-pure bulk α(2H)-, α(3R)-, and γ-In2Se3 are obtained by controlling quenching temperature, cooling conditions, and annealing duration. Ex situ annealing of bulk ingots reveals staged retained-phase evolution from α(2H) through a transient α(3R)-enriched state to γ-In2Se3, consistent with Ostwald’s rule of stages. At 673 K, the estimated α(3R) fraction in the initially α(2H) sample reaches approximately 74–78% after 2–3 h and subsequently decreases as γ-In2Se3 develops. For the two α polymorphs, the room-temperature electrical conductivity along the ∥ specimen direction increases from 2 S m–1 for α(2H)-In2Se3 to 86 S m–1 for α(3R)-In2Se3. Despite its lower carrier concentration, α(3R)-In2Se3 exhibits a substantially higher estimated mobility of 20.9 cm2 V–1 s–1, compared with 0.20 cm2 V–1 s–1 for α(2H)-In2Se3. The thermal conductivity along the same direction decreases from 3.19 to 2.70 W m–1 K–1, consistent with the lower measured sound velocities and lower calculated low-frequency phonon group velocities of α(3R)-In2Se3. These results connect thermal-history-dependent polymorph selection with electronic and phonon transport in bulk In2Se3.