scholarly journals Superionic hcp-Fe alloys and their seismic velocities in Earth’s inner core

2020 ◽  
Author(s):  
Yu He ◽  
Shichuan Sun ◽  
DuckYoung Kim ◽  
Bo Gyu Jang ◽  
He-Ping Li ◽  
...  
Keyword(s):  
Inner Core ◽  
Light Element ◽  
Seismic Velocities ◽  
Light Elements ◽  
Hexagonal Close Packed ◽  
Melting Temperatures ◽  
Superionic State ◽  
Seismological Observation ◽  
Earth’S Inner Core ◽  
Fe Alloys

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.

scholarly journals Constraints on Earth’s inner core composition inferred from measurements of the sound velocity of hcp-iron in extreme conditions

2016 ◽  
Vol 2 (2) ◽  
pp. e1500802 ◽  
Author(s):  
Tatsuya Sakamaki ◽  
Eiji Ohtani ◽  
Hiroshi Fukui ◽  
Seiji Kamada ◽  
Suguru Takahashi ◽  
...  
Keyword(s):  
Sound Velocity ◽  
Inner Core ◽  
Earth Model ◽  
Light Elements ◽  
Earth’S Core ◽  
Heated Sample ◽  
Earth's Core ◽  
The Core ◽  
Core Composition ◽  
Earth’S Inner Core

Hexagonal close-packed iron (hcp-Fe) is a main component of Earth’s inner core. The difference in density between hcp-Fe and the inner core in the Preliminary Reference Earth Model (PREM) shows a density deficit, which implies an existence of light elements in the core. Sound velocities then provide an important constraint on the amount and kind of light elements in the core. Although seismological observations provide density–sound velocity data of Earth’s core, there are few measurements in controlled laboratory conditions for comparison. We report the compressional sound velocity (VP) of hcp-Fe up to 163 GPa and 3000 K using inelastic x-ray scattering from a laser-heated sample in a diamond anvil cell. We propose a new high-temperature Birch’s law for hcp-Fe, which gives us the VP of pure hcp-Fe up to core conditions. We find that Earth’s inner core has a 4 to 5% smaller density and a 4 to 10% smaller VP than hcp-Fe. Our results demonstrate that components other than Fe in Earth’s core are required to explain Earth’s core density and velocity deficits compared to hcp-Fe. Assuming that the temperature effects on iron alloys are the same as those on hcp-Fe, we narrow down light elements in the inner core in terms of the velocity deficit. Hydrogen is a good candidate; thus, Earth’s core may be a hidden hydrogen reservoir. Silicon and sulfur are also possible candidates and could show good agreement with PREM if we consider the presence of some melt in the inner core, anelasticity, and/or a premelting effect.

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

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.

Iron–magnesium compounds under high pressure

2019 ◽  
Vol 43 (44) ◽  
pp. 17403-17407
Author(s):  
Pengyue Gao ◽  
Chuanxun Su ◽  
Sen Shao ◽  
Sheng Wang ◽  
Peng Liu ◽  
...  
Keyword(s):  
High Pressure ◽  
Inner Core ◽  
Light Element ◽  
Magnesium Compounds ◽  
Earth’S Inner Core

A dynamically and thermodynamically stable Fe-rich compound, Fe2Mg, reveals that Mg may be a light element candidate in the earth's inner core.

scholarly journals Ab Initio Thermoelasticity of Liquid Iron-Nickel-Light Element Alloys

Minerals ◽  
2020 ◽  
Vol 10 (1) ◽  
pp. 59 ◽  
Author(s):  
Hiroki Ichikawa ◽  
Taku Tsuchiya
Keyword(s):  
Ab Initio ◽  
Liquid Iron ◽  
Inner Core ◽  
Light Element ◽  
Outer Core ◽  
Light Elements ◽  
Major Element Composition ◽  
Silicon Carbon ◽  
Other Information ◽  
Core Boundary

The earth’s core is thought to be composed of Fe-Ni alloy including substantially large amounts of light elements. Although oxygen, silicon, carbon, nitrogen, sulfur, and hydrogen have been proposed as candidates for the light elements, little is known about the amount and the species so far, primarily because of the difficulties in measurements of liquid properties under the outer core pressure and temperature condition. Here, we carry out massive ab initio computations of liquid Fe-Ni-light element alloys with various compositions under the whole outer core P, T condition in order to quantitatively evaluate their thermoelasticity. Calculated results indicate that Si and S have larger effects on the density of liquid iron than O and H, but the seismological reference values of the outer core can be reproduced simultaneously by any light elements except for C. In order to place further constraints on the outer core chemistry, other information, in particular melting phase relations of iron light elements alloys at the inner core-outer core boundary, are necessary. The optimized best-fit compositions demonstrate that the major element composition of the bulk earth is expected to be CI chondritic for the Si-rich core with the pyrolytic mantle or for the Si-poor core and the (Mg,Fe)SiO3-dominant mantle. But the H-rich core likely causes a distinct Fe depletion for the bulk Earth composition.

Composition of the Earth’s inner core from high pressure sound velocity measurements of Fe-Ni-Si alloys

2020 ◽  
Author(s):  
Serena Dominijanni ◽  
Catherine McCammon ◽  
Ohtani Eiji ◽  
Ikuta Daijo ◽  
Sakamaki Tatsuya ◽  
...  
Keyword(s):  
High Pressure ◽  
Momentum Transfer ◽  
Sound Velocity ◽  
Inner Core ◽  
Light Element ◽  
Alloy Sample ◽  
Light Elements ◽  
X Ray Diffraction ◽  
X Ray ◽  
Ni Alloy

<p>Earth’s inner core likely consists of Fe-Ni alloy(s) plus a minor fraction of light element(s) to match the density and sound wave velocities of seismological models such as the preliminary reference Earth model (PREM). Among possible alloying light elements (e.g., Si, O, H, S, C), silicon is a popular candidate based on its cosmochemical abundance and potential involvement in chemical reactions at the core-mantle boundary. Previous work has shown that the solubility of Si in <em>hcp</em>-(Fe,Ni) alloy increases the stability field of the<em> bcc</em>-phase at high pressure. Comparison of sound velocity and density data of Fe-Ni-Si alloys with geophysical observations and theoretical predictions provide important constraints on the structure and dynamics of Earth’s inner core. However, knowledge of the high-pressure and high-temperature behaviour and properties of Fe-Ni alloys that contain light elements is limited. We therefore investigated <em>bcc</em>-Fe<sub>0.78</sub>Ni<sub>0.07</sub>Si<sub>0.15 </sub>alloy to compare its sound velocity and density with <em>ab initio</em> calculations and PREM in order to clarify the role of Si as a light element in Earth’s inner core.</p><p>Compressional velocities and densities of <em>bcc</em>-Fe<sub>0.78</sub>Ni<sub>0.07</sub>Si<sub>0.15</sub> alloy have been measured using inelastic X-ray scattering (IXS) and powder X-ray diffraction at the SPring-8 synchrotron facility (BL35XU beamline). High pressure was generated using a BX90-type diamond anvil cell. The metal alloy sample was loaded together with Ne (pressure medium) in a Re sample chamber and was mechanically compressed to 75 GPa through steps of 10 GPa at room temperature. IXS data were acquired at each pressure point in the range of momentum transfer of 4.24 to 7.63 nm<sup>-1</sup>. To determine density, we collected X-ray diffraction patterns of the sample before acquisition of each IXS spectrum using a flat panel detector installed in the optical system. All IXS spectra were fitted using Lorentzian functions. The dispersion relationship between energy (E) and momentum transfer (Q) was obtained by fitting all data with the following equation:</p><p>E (meV) = 4.192 x 10<sup>-4 </sup><em>v</em>p (m/s) x Q<em><sub>max</sub></em> (nm<sup>-1</sup>) x sin (π/2 x Q (nm<sup>-1</sup>)/ Q<em><sub>max</sub></em> (nm<sup>-1</sup>),</p><p>where <em>v</em>p is the sound velocity of the sample.</p><p>Preliminary results for <em>bcc</em>-Fe<sub>0.78</sub>Ni<sub>0.07</sub>Si<sub>0.15</sub> show that the energy of the longitudinal acoustic phonon increases with increasing pressure. Additionally, we found that vp follows Birch’s Law, i.e., there is a linear relationship between density and sound velocity. Based on the comparison of our results and those for <em>hcp</em>-Fe and Fe-Si alloys reported previously with PREM, we propose that <em>bcc</em>-Fe<sub>0.78</sub>Ni<sub>0.07</sub>Si<sub>0.15</sub> alloy is a viable candidate as a component of Earth’s inner core.</p><p> </p>

scholarly journals Two-step nucleation of the Earth’s inner core

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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