Interstitial hydrogen atoms in face-centered cubic iron in the Earth’s core

Ikuta, Daijo; Ohtani, Eiji; Sano‐Furukawa, Asami; Shibazaki, Yuki; Terasaki, Hidenori; Yuan, Liang; Hattori, Takanori

doi:10.1038/s41598-019-43601-z

Cited by 52 publications

(64 citation statements)

References 27 publications

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“…The latest NPD study of Fe hydride revealed that the γ-ε′phase boundary was located at temperatures 100-200 K higher than those shown in Fig. 1 19 , and the appearance of the dhcp deuteride at 673 K and 6.1 GPa was consistent with the latest result. The formation of the metastable hcp phase was sensitive to the cooling rate; a larger amount of the hcp deuteride formed at a faster cooling rate 18 .…”

Section: Resultssupporting

confidence: 82%

Crystal and Magnetic Structures of Double Hexagonal Close-Packed Iron Deuteride

Saitoh

Machida

Iizuka-Oku

et al. 2020

Sci Rep

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Neutron powder diffraction profiles were collected for iron deuteride (FeD x) while the temperature decreased from 1023 to 300 K for a pressure range of 4-6 gigapascal (GPa). The ε′ deuteride with a double hexagonal close-packed (dhcp) structure, which coexisted with other stable or metastable deutrides at each temperature and pressure condition, formed solid solutions with a composition of feD 0.68(1) at 673 K and 6.1 GPa and FeD 0.74(1) at 603 K and 4.8 GPa. Upon stepwise cooling to 300 K, the D-content x increased to a stoichiometric value of 1.0 to form monodeuteride FeD 1.0. In the dhcp FeD 1.0 at 300 K and 4.2 GPa, dissolved D atoms fully occupied the octahedral interstitial sites, slightly displaced from the octahedral centers in the dhcp metal lattice, and the dhcp sequence of close-packed fe planes contained hcp-stacking faults at 12%. Magnetic moments with 2.11 ± 0.06 μ B /fe-atom aligned ferromagnetically in parallel on the Fe planes. Iron (Fe) reacts with hydrogen (H) to form solid solution FeH x or stoichiometric monohydride FeH 1.0 at hydrogen pressures (hereafter referred to simply as pressure) in a gigapascal (GPa) range. Because of a prototypical transition-metal hydride, structural and physical properties have been intensively investigated for the Fe-H system over the past 50 years 1-20. In temperature-pressure (T-P) ranges of 0-2000 K and 0-10 GPa, three solid phases (α, ε′, and γ phases) are present: the α phase with a body-centered cubic (bcc) structure, the ε′ phase with a double hexagonal close-packed (dhcp) structure, and the γ phase with a face-centered cubic (fcc) structure 2,8,10. These phases join at a triple point at ~570 K and ~5.0 GPa 7,10. In each hydride, dissolved H atoms, partially or fully occupying the interstitial sites of a host metal lattice, cause the metal lattice to expand and provide a certain amount of electrons to the metal lattice 1,6,14,18. Thus, hydrogenation is an effective means for creating or modifying the physical properties while maintaining the structure of the metal lattice. The ε′ phase exhibits unique structural and physical properties, e.g., extensive stability, stoichiometric composition, and ferromagnetism. The phase diagram of the Fe-H system extending to 3000 K and 120 GPa indicates the ε′ phase as the only stable phase at pressures greater than 20 GPa, presumably maintaining the stoichiometric composition of FeH 1.0 12. Such unique phase stability allows for the investigation of the structural and magnetic properties over a wide T-P range. Ferromagnetic-paramagnetic transition has been experimentally investigated at ambient temperature and pressure up to 80 GPa by Mössbauer (MB) and X-ray magnetic circular dichroism (XMCD) spectroscopies 21-23. These results showed that the magnetic moment of dhcp FeH 1.0 continuously deceased with pressure and eventually disappeared at roughly 30 GPa at 300 K. The magnitude and alignment of the magnetic moments have been theoretically predicted at 0 K and ambient pressure using density-functional theory (DFT) calcu...

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Section: Resultssupporting

confidence: 82%

Crystal and Magnetic Structures of Double Hexagonal Close-Packed Iron Deuteride

Saitoh

Machida

Iizuka-Oku

et al. 2020

Sci Rep

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“…As the most abundant element in the universe, hydrogen is one of the candidates. Although its solubility in iron at atmospheric pressure (P) is very limited, the affinity of hydrogen to iron increases significantly under compression (Antonov et al, 1998;Badding et al, 1991;Fukai, 1984) as the Gibbs energy (G) of gaseous hydrogen increases rapidly with P. Solution of hydrogen in iron has been confirmed by neutron diffraction measurements (Ikuta et al, 2019;Machida et al, 2014) at high P, with H dissolving interstitially in Fe, expanding the lattice in a systematic way. The affinity to iron (siderophile behavior) at high P suggests that hydrogen is a strong candidate for the light element in the core.…”

Section: Introductionmentioning

confidence: 99%

Strong Sequestration of Hydrogen Into the Earth's Core During Planetary Differentiation

Yuan

Steinle‐Neumann

2020

Geophysical Research Letters

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We explore the partitioning behavior of hydrogen between coexisting metal and silicate melts at conditions of the magma ocean and the current core–mantle boundary with the help of density functional theory molecular dynamics. We perform simulations with the two‐phase and thermodynamic integration methods. We find that hydrogen is weakly siderophile at low pressure (20 GPa and 2,500 K), and becomes much more strongly so with pressure, suggesting that hydrogen is transported to the core in a significant amount during core segregation and is stable there. Based on our results, the core likely contains ~1 wt% H, assuming single‐stage formation and equilibration at 40 GPa. Our two‐phase simulations further suggest that silicon is entrained in the core‐forming metal, while magnesium remains in the silicate phase. This preferred incorporation of silicon hints at an explanation for the elevated Mg/Si ratio of the bulk silicate Earth relative to chondritic compositions.

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“…The EPMA analyses and the H contents are summarized in Table 1. The error in H concentration may be ±8%, which is derived mainly from uncertainty in DVH 9,28 .…”

Section: Sample Analysesmentioning

confidence: 99%

“…Fcc stoichiometric FeH included the least amounts of Si when coexisting with Fe-Si-H liquids (Fig. 3a), suggesting that Si atoms do not substitute Fe when octahedral interstitial sites are fully occupied by H atoms 28 . On the other hand, the chemical composition of the B2 phase formed in run #1 was estimated to be (Fe0.77Si0.23)H0.4, which is enriched in both Si and H than hcp (Fe0.93Si0.07)H0.25 found in run #4.…”

Section: Solid-liquid Partitioning Of Si and H The Solid/liquid Partition Coefficients Of Si And Hmentioning

confidence: 99%

“…This is because 1) only a negligible amount of H can be present in liquid/solid Fe at 1 bar and therefore H escapes from Fe lattice during decompression 6,9,26 and 2) high-pressure experiments on H-bearing systems often involve technical problems such as the failure of a diamondanvil. Recent neutron diffraction measurements are able to reveal the position and abundance of H atoms in solid Fe alloys under high pressure but so far limited to less than 12 GPa 27,28 . The properties of hcp Fe-Si-H ternary alloys, including the simultaneous solubilities of Si and H, have not been examined at high pressure and high temperature (P-T) 29 .…”

Section: Introductionmentioning

confidence: 99%

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Melting phase relations in Fe-Si-H at high pressure: Implications for Earth’s inner core crystallization and core light elements

Hikosaka

Tagawa

Hirose

et al. 2022

Preprint

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Hydrogen could be an important light element in planetary cores, but its effect on phase diagrams of iron alloys is not well known because the solubility of H in Fe is minimal at ambient pressure and high-pressure experiments on H-bearing systems have been challenging. Considering that silicon can be another major light element in planetary cores, here we performed melting experiments on the Fe-Si-H system at ~50 GPa and obtained the ternary liquidus phase relations and the solid/liquid partition coefficient, D of Si and H based on in-situ high-pressure X-ray diffraction measurements and ex-situ chemical and textural characterizations on recovered samples. Liquid crystallized hexagonal close-packed (hcp) (Fe0.93Si0.07)H0.25, which explains the observed density and velocities of the Earth’s solid inner core. The relatively high DSi = 0.94(4) and DH = 0.70(12) suggest that in addition to Si and H, the liquid outer core includes other light elements such as O, which is least partitioned into solid Fe and can thus explain the density difference between the outer and inner core. H and O, as well as Si, are likely to be major core light elements, supporting the sequestration of a large amount of water in the Earth’s core.

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Interstitial hydrogen atoms in face-centered cubic iron in the Earth’s core

Cited by 52 publications

References 27 publications

Crystal and Magnetic Structures of Double Hexagonal Close-Packed Iron Deuteride

Crystal and Magnetic Structures of Double Hexagonal Close-Packed Iron Deuteride

Strong Sequestration of Hydrogen Into the Earth's Core During Planetary Differentiation

Melting phase relations in Fe-Si-H at high pressure: Implications for Earth’s inner core crystallization and core light elements

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