Pressure-induced phase transitions of lead iodide

Abstract

Pressure is a fundamental thermodynamic variable that can efficiently modify crystal or electronic structure leading to new material states of interest. Here we search the crystal structures of lead iodide (PbI₂) up to 200 GPa using the swarm-intelligent CALYPSO structure prediction method combined with ab initio calculations. Four new stable high-pressure phases (orthorhombic Pnma, tegragonal I4/mmm, orthorhombic Immm and monoclinic C2/c) have been uncovered. A layered to three-dimensional structure transition from the ambient-pressure 2H phase to the orthorhombic Pnma phase is proposed to be taking place at ∼2.5 GPa. The newly proposed Pnma phase is a semiconductor with an indirect band gap of 1.68 eV. It is isosymmetric to the well-known cotunnite phase but with a very different bonding pattern. The semiconductor–metal transition of PbI₂ occurs at ∼27.2 GPa, where the semiconducting Pnma phase transforms to the metallic I4/mmm phase due to pressure-induced broadening of both the valance and conduction bands. The metallic nature persists in the higher-pressure phases of Immm and C2/c up to about 169.1 GPa, above which PbI₂ decomposes to its constituent elements. The current results represent a significant step toward the understanding of structural and electronic properties of PbI₂ under compression.