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Although nitrogen in the Earth’s interior has attracted significant attention recently, it remains the most enigmatic of the light elements in the Earth’s core. In this work, synchrotron X-ray diffraction (XRD) and electrical conductivity experiments were conducted on iron nitrides (Fe2N and Fe4N) in diamond anvil cells (DACs) up to about 70 ​GPa ​at ambient temperature. These results show that iron nitrides are stable up to at least 70 ​GPa. From the equation of state (EOS) parameters, iron nitrides are more compressible than iron carbides. Moreover, using the van der Pauw method and Wiedemann-Franz law, the electrical and thermal conductivity of samples were determined to be much lower than that of iron carbides. The conductivities of Fe2N and Fe4N were similar at 20–70 ​GPa, suggesting no evident effects by varying the N stoichiometries in iron nitrides. Iron nitrides are less dense and conductive but more compressible than carbides at 0–70 ​GPa. This study indicates that less nitrogen than carbon can explain geophysical phenomena in the deep Earth, such as the density deficit.

Significance Earth’s liquid outer core is mainly composed of iron alloyed with ∼8 to 10% of light elements (e.g., silicon). Convection of the liquid core generates Earth’s magnetic field, which is controlled by the thermal conductivity of the core. In this study, we investigated the resistivity and thermal conductivity of iron-silicon alloys as a candidate composition in Earth’s core via high-pressure and -temperature experiments and numerical calculations. We found a near temperature independence of the resistivity in iron-silicon alloys at Earth core’s pressure and thus a high thermal conductivity. This work indicates that if silicon is the sole major light element in Earth’s core it could depress thermal convection and promote a thermally stratified layer at the topmost outer core., Light elements in Earth’s core play a key role in driving convection and influencing geodynamics, both of which are crucial to the geodynamo. However, the thermal transport properties of iron alloys at high-pressure and -temperature conditions remain uncertain. Here we investigate the transport properties of solid hexagonal close-packed and liquid Fe-Si alloys with 4.3 and 9.0 wt % Si at high pressure and temperature using laser-heated diamond anvil cell experiments and first-principles molecular dynamics and dynamical mean field theory calculations. In contrast to the case of Fe, Si impurity scattering gradually dominates the total scattering in Fe-Si alloys with increasing Si concentration, leading to temperature independence of the resistivity and less electron–electron contribution to the conductivity in Fe-9Si. Our results show a thermal conductivity of ∼100 to 110 W⋅m−1⋅K−1 for liquid Fe-9Si near the topmost outer core. If Earth’s core consists of a large amount of silicon (e.g., > 4.3 wt %) with such a high thermal conductivity, a subadiabatic heat flow across the core–mantle boundary is likely, leaving a 400- to 500-km-deep thermally stratified layer below the core–mantle boundary, and challenges proposed thermal convection in Fe-Si liquid outer core.

Hydrogen-rich superhydrides are believed to be very promising high-Tc superconductors. Recent experiments discovered superhydrides at very high pressures, e.g. FeH5 at 130 GPa and LaH10 at 170 GPa. With the motivation of discovering new hydrogen-rich high-Tc superconductors at lowest possible pressure, here we report the prediction and experimental synthesis of cerium superhydride CeH9 at 80–100 GPa in the laser-heated diamond anvil cell coupled with synchrotron X-ray diffraction. Ab initio calculations were carried out to evaluate the detailed chemistry of the Ce-H system and to understand the structure, stability and superconductivity of CeH9. CeH9 crystallizes in a P63/mmc clathrate structure with a very dense 3-dimensional atomic hydrogen sublattice at 100 GPa. These findings shed a significant light on the search for superhydrides in close similarity with atomic hydrogen within a feasible pressure range. Discovery of superhydride CeH9 provides a practical platform to further investigate and understand conventional superconductivity in hydrogen rich superhydrides., Hydrogen-rich superhydrides are promising high-temperature superconductors which have been observed only at pressures above 170 GPa. Here the authors show that CeH9 can be synthesized at 80-100 GPa with laser heating, and is characterized by a clathrate structure with a dense 3-dimensional atomic hydrogen sublattice.