Composite number

Chemical physics

Mathematical physics

Metallurgy

Materials science

Shear (geology)

Silicon

Carbon fibers

Earth (classical element)

Thermodynamics

Hydrogen

Physics

Organic chemistry

Diffusion

Oxygen

Inner core

Molecular dynamics

Computational chemistry

Composite material

Outer core

Chemistry

Nature

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 candidates2,3, and the properties of iron–light-element alloys have been studied to constrain the IC composition4–19. Light elements have a substantial influence on the seismic velocities4–13, the melting temperatures14–17 and the thermal conductivities18,19 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&nbsp;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 IC20,21. 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. Molecular dynamics simulations show that the light elements hydrogen, oxygen and carbon become highly diffusive like liquid in solid iron under the inner-core conditions, leading to a reduction in the seismic velocities.

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