Elasticity of Hexagonal Close-Packed Iron at Earth's Inner Core Characterized

Abstract Earth’s inner core (IC) is less dense than pure iron, indicating the existence of light elements within it1. Si, S, C, O, and H have been suggested to be the candidates2,3, and the properties of Fe-light-element alloys were studied to constrain the IC composition4-19. Light elements have a significant influence on seismic velocities4-13, melting temperatures15-17, and thermal conductivities of Fe-alloys18,19. 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 IC20,21. The significant decrease in shear wave velocity (VS) gives an explanation on the soft IC21. In adddtion, the light-element convection in the IC has potential influence on the IC seismological structure and magnetic field.

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Two-phase mixture of iron–nickel–silicon alloys in the Earth’s inner core

Communications Earth & Environment ◽

10.1038/s43247-021-00298-1 ◽

2021 ◽

Vol 2 (1) ◽

Author(s):

Daijo Ikuta ◽

Eiji Ohtani ◽

Naohisa Hirao

Keyword(s):

Inner Core ◽

Earth Model ◽

Two Phase ◽

Silicon Alloys ◽

Hexagonal Close Packed ◽

Iron Nickel ◽

Earth’S Inner Core ◽

Phase Mixture ◽

Core Conditions ◽

Nickel Silicon

AbstractThe Earth’s inner core comprises iron-nickel alloys with light elements. However, there is no clarity on the phase properties of these alloys. Here we show phase relations and equations of state of iron–nickel and iron–nickel–silicon alloys up to 186 gigapascals and 3090 kelvin. An ordered derivative of the body-centred cubic structure (B2) phase was observed in these alloys. Results show that nickel and silicon influence the stability field associated with the two-phase mixture of B2 and hexagonal close-packed phases under core conditions. The two-phase mixture can give the inner core density of the preliminary reference Earth model. The compressional wave velocity of the two-phase mixture under inner core conditions is consistent with that of the preliminary reference Earth model. Therefore, a mixture of B2 and hexagonal close-packed phases may exist in the inner core and accounts for the seismological properties of the inner core such as density and velocity deficits.

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Two-step nucleation of the Earth’s inner core

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.2113059119 ◽

2022 ◽

Vol 119 (2) ◽

pp. e2113059119

Author(s):

Yang Sun ◽

Feng Zhang ◽

Mikhail I. Mendelev ◽

Renata M. Wentzcovitch ◽

Kai-Ming Ho

Keyword(s):

Molecular Dynamics ◽

Inner Core ◽

Nucleation Mechanism ◽

Body Centered Cubic ◽

Hexagonal Close Packed ◽

Key Factor ◽

Earth’S Inner Core ◽

Core Conditions ◽

Dynamics Simulations ◽

Bcc Phase

The Earth's inner core started forming when molten iron cooled below the melting point. However, the nucleation mechanism, which is a necessary step of crystallization, has not been well understood. Recent studies have found that it requires an unrealistic degree of undercooling to nucleate the stable, hexagonal, close-packed (hcp) phase of iron that is unlikely to be reached under core conditions and age. This contradiction is referred to as the inner core nucleation paradox. Using a persistent embryo method and molecular dynamics simulations, we demonstrate that the metastable, body-centered, cubic (bcc) phase of iron has a much higher nucleation rate than does the hcp phase under inner core conditions. Thus, the bcc nucleation is likely to be the first step of inner core formation, instead of direct nucleation of the hcp phase. This mechanism reduces the required undercooling of iron nucleation, which provides a key factor in solving the inner core nucleation paradox. The two-step nucleation scenario of the inner core also opens an avenue for understanding the structure and anisotropy of the present inner core.

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