Dynamic Phase Transition Leading to Extraordinary Plastic Deformability of Thermoelectric SnSe2 Single Crystal

Plastic/ductile inorganic van der Waals (vdW) thermoelectric semiconductors offer transformative advantages for high-performance flexible thermoelectric devices, which can displace the self-charge system of wearable electronics. However, the chemical origin of their plasticity remains unclear. Here, it is reported that the exceptionally large plastic strain of the bulk SnSe2 crystal results from its polytype conversion under an external force. The SnSe2 single crystal consists of a large-period polytype with 18R low-symmetry structure rather than the trigonal and hexagonal-phase that are frequently observed in the polycrystalline specimen. In situ applied pressure to the specimen drives a phase transition from low to high-symmetry, that is, from 18R to 4H, and finally to 2H-SnSe2. First principle calculations corroborate that the dynamic phase transition is a pressure-activated process and only 15 MPa pressure erases their energy gaps, consistent with experimentally measured strain-stress curves. This dynamic phase transition results in superior and near isotropic plasticity along the direction parallel and perpendicular to the cleavage plane.