  One approach to discovering topological superconductor is establishing
superconductivity based on well-identified topological insulators. However, the
coexistence of superconductivity and topological state is always arcane. In
this paper, we report how pressure tunes the crystal structure, electronic
structure, and superconductivity in topological insulator Bi2Te2.1Se0.9. The
first pressure-induced structure transition start from 8.4 GPa, followed by the
structure sequence of R-3m - C2/m - C2/c - Im-3m up to 37 GPa.
Superconductivity starts to present at 2.4 GPa with the Tc around 6.6 K.
Moreover, at around 2.5 GPa, the abnormal changes of c/a and the full width at
half maximum (FWHM) of mode indicate the occurrence of electronic topological
transition (ETT). These results suggest that ETT highly promotes the
superconductivity in topological insulator Bi2Te2.1Se0.9 in the low-pressure
rhombohedral phase. Our work clarifies the complex electronic structure in
Bi2Te2.1Se0.9 and sheds light on the new candidate for superconductivity in
topological insulators.
