Atomic-scale mechanism for shock-induced ambient-stable hexagonal close-packed Iron

Hexagonal close-packed (hcp) ε-Fe has traditionally been thought to exist only under high-pressure conditions, such as the Earth’s core. According to conventional understanding, once the pressure is released, hcp ε-Fe immediately reverts to body-centered cubic (bcc) α-Fe, and its stable presence has never been directly observed. In this study, we report the first direct atomic-scale observation of hcp ε-Fe in high-purity single-crystal Fe subjected to ultrahigh-strain-rate dynamic shock loading. Remarkably, the hcp ε-Fe phase persists stably even after pressure release, challenging the long-standing transition theory that has prevailed for the past 70 years. We propose a novel phase transition mechanism to account for this observation. These findings open new pathways for exploring the properties and potential applications of hcp ε-Fe.