First-Principles Study Of High-Pressure Phase Stability And Superconductivity Of Bi4i4

Bismuth iodide Bi4I4 exhibits intricate crystal structures and topological insulating states that are highly susceptible to influence by environments, making its physical properties highly tunable by external conditions. In this work, we study the evolution of structural and electronic properties of Bi4I4 at high pressure using an advanced structure search method in conjunction with first-principles calculations. Our results indicate that the most stable ambient-pressure monoclinic alpha-Bi4I4 phase in C2/m symmetry transforms to a trigonal P31c structure (epsilon-Bi4I4) at 8.4 GPa, then to a tetragonal P4/mmm structure (zeta-Bi4I4) above 16.6 GPa. In contrast to the semiconducting nature of ambient-pressure Bi4I4, the two high-pressure phases are metallic, in agreement with reported electrical measurements. The epsilon-Bi4I4 phase exhibits distinct ionic states of I delta- and (Bi4I4)(delta-) (delta = 0.4123 e), driven by a pressure-induced volume reduction. We show that both epsilon- and zeta-Bi4I4 are superconductors, and the emergence of pressure-induced superconductivity might be intimately linked to the underlying structural phase transitions.