Pressure-Induced Structural and Electronic Topological Transitions in Bi 1.5 Sb 0.5 Te 1.8 Se 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 A 2 B 3 metal chalcogenide TIs, Bi 1.5 Sb 0.5 Te 1.8 Se 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 1.5 Sb 0.5 Te 1.8 Se 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 …