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 ($\ensuremath{\beta}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{4}{\mathrm{I}}_{4}$) transforms across a tetragonal $P{4}_{2}/mmc$ phase at 8.5 GPa and finally to a hexagonal $P{6}_{3}/mmc$ 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 ($P{4}_{2}/mmc$ phase) to a trivial metal (the $P{6}_{3}/mmc$ phase). Our electron-phonon coupling calculations show both the $P{4}_{2}/mmc$ and $P{6}_{3}/mmc$ 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.