Pressure-induced structural and electronic transitions in bismuth iodide

Using a machine-learning accelerated crystal structural search, and ab initio calculations together with high-pressure Raman measurements, we study the crystal and electronic structures of bismuth iodide (BiI) systematically up to 50 GPa. We find that the ambient C2/m phase (beta-Bi4I4) transforms across a tetragonal P4(2)/mmc phase at 8.5 GPa and finally to a hexagonal P6(3)/mmc phase at 28.2 GPa. Our high-pressure Raman experiments identify a phase transition at around 8.6 GPa, and the experimental Raman modes evolution agrees with our calculations reasonably. Band-structures calculations suggest the BiI system undergoes a pressure-induced topological phase transition from a topological metal (P4(2)/mmc phase) to a trivial metal (the P6(3)/mmc phase). Our electron-phonon coupling calculations show both the P4(2)/mmc and P6(3)/mmc phases are superconductors and the estimated superconducting critical temperature agrees with previous measurements. Our study shows the previously reported pressure-induced superconductivity in BiI should originate from structure phase transitions.