Machine learning force field for Fe-H system and investigation on role of hydrogen on the crack propagation in α -Fe
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36 MATERIALS SCIENCE↗
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Effects of Hydrogen on the Phase Relations in Fe-FeS at Pressures of Mars-Sized Bodies
The large radius, and therefore low density, of the Martian core found in the InSight mission data analysis highlights the importance of considering other light elements besides sulfur (S), which has been considered as the main light element for Mars for decades. Hydrogen (H) is abundant in the solar system and becomes siderophile at high pressures. Although Fe-S and Fe-H systems have been studied individually, the Fe-S-H ternary system has only been investigated up to 16 GPa and 1723 K. We have investigated the Fe-S-H system at pressures and temperatures (P-T) relevant to the cores of Mars-sized planets (up to 45 GPa and well above the melting temperature of FeS) in the laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction. Here we found that sufficient hydrogen leads to the disappearance of Fe 3 S at high P-T. Instead, separate Fe-H and Fe-S phases appear at 23–35 GPa. At pressures above 35 GPa, we found a new phase appearing while Fe-S phases disappear and Fe-H phases remain. Our analysis indicates that the new phase likely contains both S and H in the crystal structure (tentatively FeS x H y where x ≈ 1 and y ≈ 1). The observed pressure-dependent changes in the phase relation may be important for understanding the structure and dynamics of the Martian core and the cores of Mars-sized exoplanets.
58 GEOSCIENCES↗
Electron Redistribution within the Nitrogenase Active Site FeMo-cofactor During Reductive Elimination of H2 to Achieve N=N Triple-Bond Activation
Nitrogen fixation by nitrogenase begins with accumulation of four reducing equivalents at the active-site FeMo-cofactor (FeMo-co), generating a state (denoted E4(4H)) with two [Fe-H-Fe] bridging hydrides. Recently, photolytic reductive elimination (re) of the E4(4H) hydrides showed that enzymatic re of E4(4H) hydride yields an H2-bound complex (E4(H2,2H)), in a process corresponding to a formal 2-electron reduction of the metal-ion core of FeMo-co. The resulting electron-density redistribution from Fe-H bonds to the metal ions themselves enables N2 to bind with concomitant H2 release, a process illuminated here by QM/MM molecular dynamics simulations. What is the nature of this redistribution? Although E4(H2,2H) hasn’t been trapped, cryogenic photolysis of E4(4H) provides a means to address this question. Photolysis of E4(4H) causes hydride-re with release of H2, generating doubly-reduced FeMo-co (denoted E4(2H)*), the extreme limit of the electron-density redistribution upon formation of E4(H2,2H). Here we examine the doubly-reduced FeMo-co core of the E4(2H)* limiting-state by 1H, 57Fe, and 95Mo ENDOR to illuminate the partial electron-density redistribution upon E4(H2,2H) formation during catalysis, complementing these results with corresponding DFT computations. Inferences from the E4(2H)* ENDOR results as extended by DFT computations include: (i) the Mo-site participates negligibly, and overall it is unlikely that Mo changes valency throughout the catalytic cycle; (ii) two distinctive E4(4H) 57Fe signals are suggested as associated with ‘anchors’ of one bridging hydride, two others with anchors of the second, with NBO-analysis identifying one anchor of each hydride as a major recipient of electrons released upon breaking Fe-H bonds.
Lukoyanov, Dmitriy↗
Reaction between Hydrogen and Ferrous/Ferric Oxides at High Pressures and High Temperatures—Implications for Sub-Neptunes and Super-Earths
Abstract Sub-Neptune exoplanets may have thick hydrogen envelopes and therefore develop a high-pressure interface between hydrogen and the underlying silicates/metals. Some sub-Neptunes may convert to super-Earths via massive gas loss. If hydrogen chemically reacts with oxides and metals at high pressures and temperatures ( P − T ), it could impact the structure and composition of the cores and atmospheres of sub-Neptunes and super-Earths. While H 2 gas is a strong reducing agent at low pressures, the behavior of hydrogen is unknown at the P − T expected for sub-Neptunes’ interiors, where hydrogen is a dense supercritical fluid. Here we report experimental results of reactions between ferrous/ferric oxides and hydrogen at 20–40 GPa and 1000–4000 K utilizing the pulsed laser-heated diamond-anvil cell combined with synchrotron X-ray diffraction. Under these conditions, hydrogen spontaneously strips iron off the oxides, forming Fe-H alloys and releasing oxygen to the hydrogen medium. In a planetary context where this reaction may occur, the Fe-H alloy may sink to the metallic part of the core, while released oxygen may stabilize as water in the silicate layer, providing a mechanism to ingas hydrogen to the deep interiors of sub-Neptunes. Water produced from the redox reaction can also partition to the atmosphere of sub-Neptunes, which has important implications for understanding the composition of their atmospheres. In addition, super-Earths converted from sub-Neptunes may contain a large amount of hydrogen and water in their interiors (at least a few wt% H 2 O). This is distinct from smaller rocky planets, which were formed relatively dry (likely a few hundredths wt% H 2 O).
58 GEOSCIENCES↗
The stability of FeH x and hydrogen transport at Earth’s core mantle boundary
Iron hydride in Earth’s interior can be formed by the reaction between hydrous minerals (water) and iron. Studying iron hydride improves our understanding of hydrogen transportation in Earth’s interior. Our high-pressure experiments found that face-centered cubic (fcc) FeH x (x≤1) is stable up to 165 GPa, and our ab initio molecular dynamics simulations predicted that fcc FeH x transforms to a superionic state under lower mantle conditions. In the superionic state, H-ions in fcc FeH become highly diffusive-like fluids with a high diffusion coefficient of ~3.7 × 10 -4 cm 2 s -1 , which is comparable to that in the liquid Fe-H phase. The densities and melting temperatures of fcc FeH x were systematically calculated. Similar to superionic ice, the extra entropy of diffusive H-ions increases the melting temperature of fcc FeH. Further, the wide stability field of fcc FeH enables hydrogen transport into the outer core to create a potential hydrogen reservoir in Earth’s interior, leaving oxygen-rich patches (ORP) above the core mantle boundary (CMB).
58 GEOSCIENCES↗
Solar wind Fe and CNO measurements in high-speed flows
Solar wind characteristics in driver plasma and coronal hole-associated flow types are analyzed. Measurements of solar wind Fe charge states and densities in well-defined driver plasma and coronal hole-associated high-speed streams, and charge distributions of CNO ions in high speed streams collected with the ultra low energy charge analyzer on ISEE 3 are examined. The Fe-H velocity differences and Fe/H abundance ratios are studied. The data reveal that the driver plasma solar wind has charge states of 15 or 16 with a coronal temperature = 4 x 10 to the 6th K, and the Fe charge states distributions in coronal hole-associated streams = 9 or 10 with a coronal temperature = 1.4 x 10 to the 6th K; the ionization temperature for the CNO group = (1.3 + or - 0.3) x 10 to the 6th K.
Ipavich, F. M.↗
Materials Data on FeH by Materials Project
FeH is Halite, Rock Salt structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Fe is bonded to six equivalent H atoms to form a mixture of corner and edge-sharing FeH6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Fe–H bond lengths are 1.88 Å. H is bonded to six equivalent Fe atoms to form a mixture of corner and edge-sharing HFe6 octahedra. The corner-sharing octahedral tilt angles are 0°.
36 MATERIALS SCIENCE↗
Materials Data on FeH3 by Materials Project
FeH3 is Uranium Silicide structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Fe3+ is bonded to twelve equivalent H1- atoms to form a mixture of face and corner-sharing FeH12 cuboctahedra. All Fe–H bond lengths are 1.85 Å. H1- is bonded in a square co-planar geometry to four equivalent Fe3+ atoms.
36 MATERIALS SCIENCE↗
Materials Data on FeH4 by Materials Project
FeH4 crystallizes in the monoclinic P2_1/m space group. The structure is three-dimensional. Fe2+ is bonded in a 9-coordinate geometry to eleven H+0.50- atoms. There are a spread of Fe–H bond distances ranging from 1.59–2.12 Å. There are three inequivalent H+0.50- sites. In the first H+0.50- site, H+0.50- is bonded in a distorted square co-planar geometry to four equivalent Fe2+ atoms. In the second H+0.50- site, H+0.50- is bonded in a distorted bent 120 degrees geometry to two equivalent Fe2+ atoms. In the third H+0.50- site, H+0.50- is bonded in a 1-coordinate geometry to three equivalent Fe2+ atoms.
36 MATERIALS SCIENCE↗