First-principles study of high-pressure phase stability and superconductivity of Bi 4 I 4

Abstract

Bismuth iodide ${\\mathrm{Bi}}_{4}{\\mathrm{I}}_{4}$ 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 ${\\mathrm{Bi}}_{4}{\\mathrm{I}}_{4}$ 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 $\\ensuremath{\\alpha}\\ensuremath{-}{\\mathrm{Bi}}_{4}{\\mathrm{I}}_{4}$ phase in $C2/m$ symmetry transforms to a trigonal $P31c$ structure ($\\ensuremath{\\varepsilon}\\ensuremath{-}{\\mathrm{Bi}}_{4}{\\mathrm{I}}_{4}$) at 8.4 GPa, then to a tetragonal $P4/mmm$ structure ($\\ensuremath{\\zeta}\\ensuremath{-}{\\mathrm{Bi}}_{4}{\\mathrm{I}}_{4}$) above 16.6 GPa. In contrast to the semiconducting nature of ambient-pressure ${\\mathrm{Bi}}_{4}{\\mathrm{I}}_{4}$, the two high-pressure phases are metallic, in agreement with reported electrical measurements. The $\\ensuremath{\\varepsilon}\\ensuremath{-}{\\mathrm{Bi}}_{4}{\\mathrm{I}}_{4}$ phase exhibits distinct ionic states of ${\\mathrm{I}}^{\\ensuremath{\\delta}\\ensuremath{-}}$ and (${\\mathrm{Bi}}_{4}{\\mathrm{I}}_{3}{)}^{\\ensuremath{\\delta}+}$ ($\\ensuremath{\\delta}=0.4123$ e), driven by a pressure-induced volume reduction. We show that both $\\ensuremath{\\varepsilon}$- and $\\ensuremath{\\zeta}\\ensuremath{-}{\\mathrm{Bi}}_{4}{\\mathrm{I}}_{4}$ are superconductors, and the emergence of pressure-induced superconductivity might be intimately linked to the underlying structural phase transitions.