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Abstract Hexagonal close‐packed (hcp) structured Fe‐Ni alloy is believed to be the dominant phase in the Earth's inner core. This phase is expected to contain 4%–5% light elements, such as Si and H. While the effects of individual light element candidates on the equation of state (EoS) of the hcp Fe metal have been studied, their combined effects remain largely unexplored. In this study, we report the equations of state for two hcp‐structured Fe‐Si‐H alloys, namely Fe_0.83Si_0.17H_0.07and Fe_0.83Si_0.17H_0.46, using synchrotron X‐ray diffraction measurements up to 125 GPa at 300 K. These alloys were synthesized by cold compression of Fe‐9wt%Si in either pure H₂or Ar‐H₂mixture medium in diamond‐anvil cells. The volume increase caused by a H atom in hcp Fe‐Si‐H alloys is approximately eight times greater than that by a Si atom. We used the improved data set to develop a composition‐dependent EoS that covers a wide range of compositions. Our calculated density and bulk sound velocity of hcp Fe‐Si‐H alloys suggest a large trade‐off between Si and H contents in fitting the seismic properties of the inner core. Combining our new EoS with geophysical and geochemical constraints, we propose 1.6–3 wt% Si and 0.15–0.6 wt% H in the Earth's inner core. 

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 H₂gas is a strong reducing agent at low pressures, the behavior of hydrogen is unknown at theP−Texpected 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% H₂O). This is distinct from smaller rocky planets, which were formed relatively dry (likely a few hundredths wt% H₂O). 

Many sub-Neptune exoplanets have been believed to be composed of a thick hydrogen-dominated atmosphere and a high-temperature heavier-element-dominant core. From an assumption that there is no chemical reaction between hydrogen and silicates/metals at the atmosphere–interior boundary, the cores of sub-Neptunes have been modeled with molten silicates and metals (magma) in previous studies. In large sub-Neptunes, pressure at the atmosphere–magma boundary can reach tens of gigapascals where hydrogen is a dense liquid. A recent experiment showed that hydrogen can induce the reduction of Fe ${}^{2+}$in (Mg,Fe)O to Fe ${}^0$metal at the pressure–temperature conditions relevant to the atmosphere–interior boundary. However, it is unclear whether Mg, one of the abundant heavy elements in the planetary interiors, remains oxidized or can be reduced by H. Our experiments in the laser-heated diamond-anvil cell found that heating of MgO + Fe to 3,500 to 4,900 K (close to or above their melting temperatures) in an H medium leads to the formation of Mg ${}_2$FeH ${}_6$and H ${}_2$O at 8 to 13 GPa. At 26 to 29 GPa, the behavior of the system changes, and Mg–H in an H fluid and H ${}_2$O were detected with separate FeH ${}_x$. The observations indicate the dissociation of the Mg–O bond by H and subsequent production of hydride and water. Therefore, the atmosphere–magma interaction can lead to a fundamentally different mineralogy for sub-Neptune exoplanets compared with rocky planets. The change in the chemical reaction at the higher pressures can also affect the size demographics (i.e., “radius cliff”) and the atmosphere chemistry of sub-Neptune exoplanets. 

The Earth’s core–mantle boundary presents a dramatic change in materials, from silicate to metal. While little is known about chemical interactions between them, a thin layer with a lower velocity has been proposed at the topmost outer core (Eʹ layer) that is difficult to explain with a change in concentration of a single light element. Here we perform high-temperature and -pressure laser-heated diamond-anvil cell experiments and report the formation of SiO2 and FeHx from a reaction between water from hydrous minerals and Fe–Si alloys at the pressure–temperature conditions relevant to the Earth’s core–mantle boundary. We suggest that, if water has been delivered to the core–mantle boundary by subduction, this reaction could enable exchange of hydrogen and silicon between the mantle and the core. The resulting H-rich, Si-deficient layer formed at the topmost core would have a lower density, stabilizing chemical stratification at the top of the core, and a lower velocity. We suggest that such chemical exchange between the core and mantle over gigayears of deep transport of water may have contributed to the formation of the putative Eʹ layer.