Progress in the research of superionic substances in the earth's core丨This Nature"

The superionic state is between the solid and liquid states, and in the superionic state of the substance, some ions move as fast as liquids, while the other part of the ions are fixed in the material structure like a "skeleton". In 1988, it was predicted that ice would turn into a superionic state at high temperatures and pressures. The electrical conductivity of superionic ice is close to that of metals, which may exist inside and affect the magnetic fields of Uranus and Neptune. The superionary state is a new state of matter in earth and planetary science, which has attracted widespread attention due to its special properties.

The team of Li Heping and He Yu, researchers of the Key Laboratory of High Temperature and Pressure of Materials in the Earth's Interior, Institute of Geochemistry, Chinese Academy of Sciences, and the team of Beijing High Pressure Science Center collaborated to calculate and simulate the properties of various ferroalloys at the temperature and pressure of the earth's core. The study found that the hexagonal phase iron-hydrogen, iron-carbon and ferro-oxygen alloys were converted to a superionic state at the temperature of the core, indicating that the earth's core is not a solid state as traditionally recognized, but a superionic state composed of solid iron and flowing light elements. On February 10, the results of the research were published online in Nature.

The Earth's core is in an extreme state of high temperature and pressure, and the First Principles Molecular Dynamics method based on density functional theory is an important research method to simulate the state and properties of matter deep in the Earth. Based on previous research results, the contents of hydrogen, carbon and oxygen in the selected model are 0.45 wt%, 1.33 wt% and 1.75 wt% respectively (corresponding to the molecular formulas FeH0.25, FeC 0.0625, FeO0.0625). Studies have shown that the above alloys are converted to a superionic state when the temperature rises to 2500-3000 K, and in the superionic alloy, hydrogen, carbon, and oxygen ions diffuse rapidly in the gaps of the iron "skeleton" structure, showing the characteristics of the fluid. In view of the overheating state that is easy to obtain in the calculation simulation, that is, the computational model remains solid above the melting point, the team used the solid-liquid coexistence method to constrain the melting point of the above alloy, and calculated that the melting point of the superionic alloy under the core pressure is about 5500-6000 K lower than that of pure iron. Through a series of computational simulations at temperature and pressure, the solid-superionic-liquid transition phase diagram was obtained, which confirmed the stability of the superionic alloy under the temperature and pressure of the core.

The diffusion coefficients of carbon, hydrogen and oxygen ions in ferroalloys increase with temperature, and under the condition of core boundary, their diffusion coefficients are almost unchanged in solids and melts, indicating that the diffusion properties of light elements have not changed significantly in the inner and outer nuclei, so the convection of light elements may be widely present in the kernel. Rapidly diffusing ions may lead to changes in conductivity, and according to calculations, the ion conductivity of superionic ferroalloys is found to be 2-3 orders of magnitude lower than that of electrons, so the change in conductivity caused by superionic state transition is almost negligible.

The research team simulated the elastic properties and seismic wave velocity of the superionic alloy, which fully considered the ion diffusion and non-harmonic vibration of the lattice, and was suitable for superionic system. The study found that the superion state transition led to the accelerated softening of the alloy, causing a significant decrease in seismic wave velocity, and its values could be well consistent with seismological observations. The simulation results show that the flow of light element impurities can cause the softening of ferroalloys, especially the reduction of the shear wave velocity explains the mystery of kernel softening.

Seismological studies have shown that the core structure exhibits complex heterogeneous and anisotropic features, as well as characteristics such as seismic wave attenuation and structural changes. Dissecting the above unsolved mysteries is the key to the structure, composition, and evolution of the cognitive core. The hyperionic core, updated the understanding of the state of the core, the light elements of the general motion of fluids provide new clues for understanding the core convection, anisotropic structure and formation, the relationship between the earth's core structure and the geomagnetic field, etc.

The research work has been funded by the Strategic Leading Science and Technology Project of the Chinese Academy of Sciences, the National Natural Science Foundation of China, and the Youth Innovation Promotion Association of the Chinese Academy of Sciences.

Progress has been made in the study of superioneous substances in the Earth's core

Source: Institute of Geochemistry, Chinese Academy of Sciences