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.

The Earth's core consists of iron as the major component. The melting point of iron at the inner core boundary constrains the thermal structure and solidification of the Earth's core. However, the current estimation of the melting temperature of iron under the core conditions has significant variations. Here, we measured the temperatures of iron shocked up to ~256 GPa using precise pyrometer and velocimeter diagnostics via a two‐stage light‐gas gun. Our results indicated that the melting temperatures of iron at the core‐mantle and inner core boundaries are 4300(250) and 5950(400) K, respectively. These temperatures are significantly lower than some previous shock experiments but are overall consistent with the recent results determined by fast X‐ray diffraction techniques, X‐ray absorption experiments in laser‐heated diamond anvil cells, and by ab initio computations. Our iron melting curve indicates a relatively small Clapeyron slope and supports thermal models for a young inner core.

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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.

The solid inner core of the Earth is growing out of the liquid outer core, primarily composed of iron with 5%-10% nickel and a small number of light elements (Hirose et al., 2013). The inner core is expected to contain fewer light elements as they prefer staying in the liquid outer core. Those elements have a great influence on the melting behavior of iron (e.g., Liu et al., 2016;Morard et al., 2008Morard et al., , 2017. In general, the incorporation of light elements such as oxygen, silicon, and sulfur could depress the melting point of iron by up to 700 K at the inner core boundary (ICB) (Alfè et al., 2002). It is thus expected that the upper bound of the temperature at the ICB is the melting point (T m ) of iron at 330 GPa, while the lower bound depends on the identity and concentration of light elements. That is, the T m values of iron and iron alloys at high pressure present the key to decode the temperature distribution of the Earth's core as well as the evolution and dynamics related to this region, including heat budget and the generation of magnetic field, heat flow across the ICB and the core-mantle boundary (CMB), and the crystal structure and density deficit of the inner core (Fei et al., 2016;Vočadlo et al., 1999).Extensive studies have been dedicated to constrain the T m of highly compressed iron including dynamic and static high-pressure experiments (e.g.,

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