Superionic iron alloys and their seismic velocities in Earth's inner core

Earth's inner core (IC) is less dense than pure iron, indicating the existence of light elements within it¹. Silicon, sulfur, carbon, oxygen and hydrogen have been suggested to be the candidates^2,3, and the properties of iron-light-element alloys have been studied to constrain the IC composition^4-19. Light elements have a substantial influence on the seismic velocities^4-13, the melting temperatures^14-17 and the thermal conductivities^18,19 of iron alloys. However, the state of the light elements in the IC is rarely considered. Here, using ab initio molecular dynamics simulations, we find that hydrogen, oxygen and carbon in hexagonal close-packed iron transform to a superionic state under the IC conditions, showing high diffusion coefficients like a liquid. This suggests that the IC can be in a superionic state rather than a normal solid state. The liquid-like light elements lead to a substantial reduction in the seismic velocities, which approach the seismological observations of the IC^20,21. The substantial decrease in shear-wave velocity provides an explanation for the soft IC²¹. In addition, the light-element convection has a potential influence on the IC seismological structure and magnetic field.