Experimental evidence for superionic Fe–C alloy revealed by shear softening in Earth’s inner core
Yuqian Huang, Yu He, Youjun Zhang, Jun Li, Hao Long, Bo Gan, Gang Jiang, Qiang Wu, Ho‐kwang Mao
Abstract
ABSTRACT Seismic observations reveal that Earth’s inner core is rigid yet displays ultralow shear wave velocities (Vs) 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/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 the 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 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.