Pressure induced phase transitions in topological crystalline insulator SnTe and its comparison with semiconducting SnSe: Raman and First-principles studies

We report Raman study of a topological crystalline insulator (TCI) SnTe and a normal semiconductor SnSe, as a function of pressure at room temperature along with first-principles density functional theory calculations. Under pressure, iso-structural transition is observed in SnTe as revealed by the anomalous softening of the strongest Raman mode up to 1.5 GPa. Further, SnTe undergoes structural transitions at ~ 5.8, ~ 12 and ~ 18.3 GPa. The 5.8 GPa transition is associated with a structural transition from ambient cubic (Fm-3m) to orthorhombic (Pnma) phase which is no longer a topological insulator. Above the transition pressure of 12 GPa another orthorhombic Pnma[GeS] phase is stabilized coexisting with Pnma phase. Above 18.3 GPa enthalpy calculations show a transition from orthorhombic Pnma to a more symmetric cubic (Pm -3m) structure. Our high-pressure study of SnSe on the other hand reveals that it undergoes two phase transitions: one from the orthorhombic (Pnma) structure to orthorhombic (Cmcm) structure at ~ 6.2 GPa and the other at ~ 12.9 GPa in which the Cmcm phase undergoes a semi-metal to metal transition. Density functional theory calculations capture the contrast in pressure dependent behaviour of topological crystalline insulator SnTe and a normal semiconductor SnSe.