Phonon signatures of multiple topological quantum phase transitions in compressed TlBiS2: A combined experimental and theoretical study

We report the hydrostatic pressure induced two topological phase transitions in strong spin-orbit coupled material TlBiS 2 at room temperature. Frequencies of the A 1g and E g phonons are observed to increase monotonically up to ∼4.0 GPa, but with a clear slope change in A 1g mode at ∼1.8 GPa. Interestingly, there are two clear anomalies noticed in phonon linewidths of E g mode at pressures ∼0.5 and ∼1.8 GPa. Such anomalies are evidence of isostructural electronic transitions associated with unusual electron-phonon coupling. The high-pressure synchrotron powder diffraction and Raman show a first-order phase transition above 4 GPa. First-principles density functional theory-based calculations of electronic band structure, topological invariant Z 2 and mirror Chern number n M reveal that the phonon anomalies at ∼0.5 and ∼1.8 GPa are linked to the band inversions at and F points of the Brillouin zone respectively. The first band inversion at point at ∼0.5 GPa changes the Z 2 from 0 to 1 leading to the transition of TlBiS 2 system into a topological insulator. The second band inversion at F point at ∼1.8 GPa results in n M = 2, revealing a transition to a topological crystalline insulating state. Therefore the applied pressure systematically tunes the electronic states of TlBiS 2 from a normal semiconductor to a topological insulator and finally into a topological crystalline insulator at two distinct pressures of ∼0.5 and ∼1.8 GPa respectively, before undergoing a structural phase transition at ∼4 GPa.