Pressure-induced phase transitions in Bi 4 I 4

A first-principles study is performed to systematically investigate the structural evolution, the stability, mechanical and electronic properties of β- and ε-Bi 4 I 4 under high pressure. Using the Gibbs free energy method, the transition pressure for the structural transformation from monoclinic β-Bi 4 I 4 to the rhombohedral ε-phase was determined to be 5.25 GPa at 0 K. Furthermore, the β- to ε- phase transition is associated with a discontinuous volume change of 6.81%, which is consistent with the characteristics of a first-order phase transition. Mechanical stability analysis reveals that β-Bi 4 I 4 becomes mechanically unstable above 4 GPa, providing an indirect indication of structural instability near the predicted transition region. Phonon calculations reveal distinct pressure-dependent lattice dynamic behaviors for the two phases. Regarding mechanical performance, the β-phase exhibits a pressure-induced brittle-to-ductile transition at approximately 1.6 GPa, with ductility progressively enhancing up to the phase transition point. The electronic structure of β-Bi 4 I 4 exhibits a pressure-induced band-gap reduction, with the band gap gradually decreasing under compression and eventually closing at higher pressure. In contrast, the high-pressure ε-phase remains metallic throughout the investigated pressure range. These results provide a more detailed description of the pressure-induced structural, mechanical, and electronic evolution of Bi 4 I 4 .