  Hydrogen-rich superhydrides are believed to be very promising high-Tc
superconductors as they are expected to mimic characteristics of metallic
hydrogen. 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
below 100 GPa in the laser-heated diamond anvil cell. Ab-initio calculations
were carried 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 substantially dense
3-dimensional hydrogen sublattice at 100 GPa. These findings shed a new light
on the search for superhydrides in close proximity 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.
