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

Yu He (), Shichuan Sun, Duck Young Kim, Bo Gyu Jang, Heping Li and Ho-kwang Mao
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Yu He: Chinese Academy of Sciences
Shichuan Sun: Chinese Academy of Sciences
Duck Young Kim: Center for High Pressure Science and Technology Advanced Research
Bo Gyu Jang: Center for High Pressure Science and Technology Advanced Research
Heping Li: Chinese Academy of Sciences
Ho-kwang Mao: Center for High Pressure Science and Technology Advanced Research

Nature, 2022, vol. 602, issue 7896, 258-262

Abstract: Abstract 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 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.

Date: 2022
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DOI: 10.1038/s41586-021-04361-x

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