Synthesis of clathrate cerium superhydride CeH9 at 80-100 GPa with atomic hydrogen sublattice

Hydrogen-rich superhydrides are believed to be very promising high-T_c 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-T_c 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.