Phase transition of Fe under extreme conditions studied by using an anharmonic phonon approach based on machine learning force fields

The phase transition structure and dynamical mechanisms of solid-state bcc phase iron (Fe) under extreme conditions remain an open question. This study systematically investigates the phase transition process and dynamical mechanisms of solid Fe at 0-1000 K and 0-30 GPa by combining machine learning force field molecular dynamics simulations and an anharmonic phonon approach. Considering the high-temperature anharmonic effects, we calculated and compared the Helmholtz and Gibbs free energies of bcc, hcp, and fcc phase Fe. At zero temperature, Fe transitions from the bcc phase to the hcp phase at 13.83 GPa. Due to the influence of temperature anharmonic effects, this transition pressure increases with rising temperature, reaching 17.20 GPa at 1000 K. During the bcc → hcp phase transition, the Gibbs free energy of the fcc phase is always higher than that of the bcc or hcp phases, indicating that the fcc phase is a metastable phase. The transverse acoustic branch (TA1) is the most sensitive to temperature and pressure, exhibiting frequency softening phenomena during the phase transition, which is the origin of the dynamic instability and strong phonon anharmonicity of the bcc phase. According to the phonon vibration polarization vector, the vibrational modes of the TA1 mode near the Γ point provide a continuous phase transition geometric pathway for the bcc phase to transition to the hcp phase through the intermediate fcc phase. These theoretical results support the experimental two-step phase transition viewpoint of Fe from bcc to hcp under high temperature and high pressure.