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

SnTe is a narrow band-gap topological crystalline insulator (TCI), whereas SnSe is a normal semiconductor. We report Raman study of SnTe and SnSe as a function of pressure at room temperature along with first-principles density functional theory calculations. Under pressure, isostructural transition is observed in SnTe, as revealed by the anomalous softening of the strongest Raman mode up to 1.5 GPa, accompanied by an increase in the linewidth. Our first-principles calculations show that the mirror Chern number of SnTe does not change and the TCI phase remains unaffected by pressure. Raman signatures of its phase transition at 1.5 GPa are associated with phonon instability at the $\mathrm{\ensuremath{\Gamma}}$ point and inversion of the lowest-energy conduction bands. An anomaly in the electron-phonon coupling results in anomalous behavior of the Raman modes at this pressure. Further, SnTe undergoes structural transitions at $\ensuremath{\sim}5.8, \ensuremath{\sim}12$, and $\ensuremath{\sim}18.3\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The 5.8-GPa transition is associated with a structural transition from the ambient cubic ( $Fm\overline{3}m$ ) to orthorhombic (Pnma) phase, which is no longer a topological insulator, resulting in a topological phase transition. Above the transition pressure of 12 GPa, another orthorhombic Pnma[GeS] phase is stabilized, coexisting with the Pnma phase. The reduction in the number of observed Raman modes above $\ensuremath{\sim}18.3\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and enthalpy calculations show a transition from orthorhombic (Pnma) to a more symmetric cubic ($\mathrm{Pm}\overline{3}\mathrm{m}$) 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 the orthorhombic (Cmcm) structure at $\ensuremath{\sim}6.2\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and the other at $\ensuremath{\sim}12.9\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, in which the Cmcm phase undergoes a semimetal to metal transition. Density functional theory calculations capture the contrast in the pressure-dependent behavior of the topological crystalline insulator SnTe and the normal semiconductor SnSe.