Hydrogen‐Dominated Iron Phase Stability and the Structure of the Earth's Inner Core

Abstract The crystal structure of Earth's inner core (IC) remains debated, with the free energy difference between hexagonal close‐packed (hcp) and body‐centered cubic (bcc) iron under IC conditions being very small. We use ab initio and machine learning force field molecular dynamics simulations to show that hydrogen can strongly influence the phase stability of inner‐core iron. At 330 GPa and 6,000 K, ∼1.4 at.% hydrogen is sufficient to overcome the small free‐energy difference between pure iron phases and favor bcc phase stability. At geochemically plausible IC H contents of 0–11.3 at.%, hcp, bcc or mixed phases can be stable, with bcc phases dominant above 2.78 at.% hydrogen. Depth‐dependent phase transitions across 330–360 GPa can arise from variations in hydrogen content and temperature, suggesting that these two factors are important controls on Fe–H phase stability and may provide a possible mechanism for interpreting the IC's complex seismic features.