Pressure-Induced Structural and Electronic Topological Transitions in Bi_1.5Sb_0.5Te_1.8Se_1.2 Alloy

Topological insulators (TIs) have surface states that are topologically protected against scattering or defects, and thus have been the subject of intense research. Among the A₂B₃ metal chalcogenide TIs, Bi_1.5Sb_0.5Te_1.8Se_1.2 quaternary alloy has been reported to have lowest bulk conductivity, being an optimal composition to study the surface transport properties. Here, using combined theoretical and experimental investigations, we show the hydrostatic pressure effects on the structural, vibrational and topological properties of the Bi_1.5Sb_0.5Te_1.8Se_1.2. Within pressure range up to 45 GPa, two structural phase transitions were observed; from R-3m phase to C2/m phase at ~13 GPa, and to disordered I4/mmm phase at ~22 GPa. Within the R-3m phase, several electronic transitions were also observed. Indirect bulk band gap transited to direct bulk band gap at ~5.8 GPa, and bulk gap closed with an appearance of Dirac semimetal (DSM) state at ~8.2 GPa. Anomaly in Full Width at Half Maximum (FWHM) of in-plane Raman peaks and c/a lattice constant ratio suggests contribution of electron-phonon coupling to the DSM transition.