The structure and stability of iron near melting at multi-megabar pressures are of significant interest in high-pressure physics and earth and planetary sciences. While the body-centered cubic (bcc) phase is generally recognized as unstable at lower temperatures, its stability relative to the hexagonal close-packed (hcp) phase at high temperatures (approximately 0.5 eV) in the Earth's inner core (IC) remains a topic of ongoing theoretical and experimental debate. Motivated by a series of studies attempting to understand the state of the IC and resulting in contradictory findings, we aim to explore the physical factors that may contribute to this confusion in a broader context. Our ab initio calculations show a significant drop in energy, the emergence of a plateau and a local minimum in the potential energy surface, and stabilization of all phonon modes of bcc iron at elevated electron temperatures ($>$1–1.5 eV). These effects increase the competition among the bcc, hcp, and face-centered cubic (fcc) phases and lead to the metastability of the bcc structure. Furthermore, at pressure-temperature conditions relevant to the IC, we can expect substantial coexistence of fcc and hcp iron due to their similar free energies and low transition barrier ($\sim 0.2$ eV/atom), and the thermodynamic stability of bcc iron is enhanced by its substantial lattice vibration entropy. These findings provide a clear theoretical framework for understanding iron phase relations and solidification processes, both experimentally and in the IC.

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• High-pressure studies
• Phase transitions
• Potential energy surfaces
• Thermodynamics
• Transition temperature
• Elemental metals
• Planetary plasmas
• First-principles calculations