Superionic hcp-Fe alloys and their seismic velocities in Earth’s inner core

Abstract Earth’s inner core (IC) is less dense than pure iron, indicating the existence of light elements within it 1 . Si, S, C, O, and H have been suggested to be the candidates 2,3 , and the properties of Fe-light-element alloys were studied to constrain the IC composition 4-19 . Light elements have a significant influence on seismic velocities 4-13 , melting temperatures 15-17 , and thermal conductivities of Fe-alloys 18,19 . However, the state of the light elements in the IC is rarely considered. Using ab initio molecular dynamics (AIMD) simulations, we found that H, O, and C in hexagonal close-packed (hcp) Fe transform to a superionic state under IC conditions, showing high diffusion coefficients like liquid. It suggests the IC can be in superionic state rather than normal solid state. The liquid-like light elements lead to a significant reduction in the seismic velocities approaching the seismological observation of the IC 20,21 . The significant decrease in shear wave velocity (V S ) gives an explanation on the soft IC 21 . In adddtion, the light-element convection in the IC has potential influence on the IC seismological structure and magnetic field.