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 (V s) and an ultra-high 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-1.5 wt% C (Fe-1.5C) alloy under core-like conditions. Theoretical simulations reveal a temperature-induced transition to a superionic state in the hcp-Fe-1.5C alloy above T/T m ≈ 0.68 (T m: melting temperature) under high-pressure conditions. This transition provides a physical explanation for both the ∼23% reduction in V s relative to pure Fe and the elevated Poisson's ratio (∼0.43) measured for the Fe-1.5C alloy at 140 GPa and T/T m ≈ 0.83 in our shock-wave experiments. Our findings suggest that the inner core may have liquid-like softness arising from both the superionic diffusion of light elements and the 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.