Local insight to the structural phase transition sequence of Bi₂Se₃under quasi-hydrostatic and nonhydrostatic pressure

High-pressure behaviors of Bi₂Se₃, as one of layered 3D topological insulators, has attracted tremendous interest recent years. However, the phase transition sequence of Bi₂Se₃remain controversial. In this work, we explore the structural phase sequence of topological insulator Bi₂Se₃using high-pressure x-ray absorption fine structure (XAFS) spectroscopy under quasi-hydrostatic and nonhydrostatic pressure up to 42 GPa. By examining the XAFS features, we find that the appearance ofC2/cphase of Bi₂Se₃is strongly dependent on pressure condition,C2/cphase of Bi₂Se₃only exists under quasi-hydrostatic pressure condition. The phonon dispersion calculations also show thatC2/cphase is dynamic unstable. Furthermore, we confirm that Bi₂Se₃possessesI4/mmmphase rather thanIm-3mand 9/10-foldC2/mphase at high pressure. Combining the experimental and theoretical results, we determine the structural phase transition sequence for Bi₂Se₃ofR-3m→C2/m→C2/c→I4/mmmphase under quasi-hydrostatic pressure condition andR-3m→C2/m→I4/mmmphase under nonhydrostatic pressure condition. The dynamic unstability and pressure condition sensitivity ofC2/cphase may be account for the absence ofC2/cphase in the phase transition sequence under nonhydrostatic pressure condition. Our findings obtain the high-pressure phase transition sequences of Bi₂Se₃under hydrostatic and nonhydrostatic pressure condition, which can facilitate researchers to explore the novel properties in layered 3D topological insulators.