Experimental Evidence for Superionic Fe-C Alloy Revealed by Shear Softening in Earth’s Inner Core

Seismic observations reveal that Earth’s inner core is rigid yet displays ultralow shear wave velocities (Vs) and an ultrahigh Poisson’s ratio (∼0.45). While superionic behavior in iron-light element alloys (e.g., carbon) has been proposed to explain these features, experimental validation has been lacking. Here, we combine dynamic high pressure-temperature (P-T) experiments with ab initio molecular dynamics simulations to investigate the elastic properties and atomic behaviors of hexagonal close-packed (hcp) Fe-1.5wt.%C alloy under core-like conditions. Theoretical simulations reveal a temperature-induced transition to a superionic state in hcp-Fe-1.5C alloy above T/Tm ≈ 0.68 (Tm: melting temperature) under high-pressure conditions. This transition provides a physical explanation for both the ∼23% reduction in Vs relative to pure Fe and the elevated Poisson’s ratio (∼0.43) measured for Fe-1.5C alloy at 140 GPa and T/Tm ≈ 0.83 in our shock-wave experiments. Our findings suggest that the inner core may behave liquid-like softness arising from both superionic diffusion of light elements and atomic collective motion of iron atoms. This work experimentally confirms a previously unrecognized state of matter in the inner core, reconciling long-standing geophysical and seismological discrepancies.