  The research on hydrogen-rich ternary compounds attract tremendous attention
for it paves new route to room-temperature superconductivity at lower
pressures. Here, we study the crystal structures, electronic structures, and
superconducting properties of the ternary Ca-U-H system, combining crystal
structure predictions with ab-initio calculations under high pressure. We found
four dynamically stable structures with hydrogen clathrate cages: CaUH12-Cmmm,
CaUH12-Fd-3m, Ca2UH18-P-3m1, and CaU3H32-Pm-3m. Among them, the Ca2UH18-P-3m1
and CaU3H32-Pm-3m are likely to be synthesized below 1 megabar. The f electrons
in U atoms make dominant contribution to the electronic density of states
around the Fermi energy. The electron-phonon interaction calculations reveal
that phonon softening in the mid-frequency region can enhance the
electron-phonon coupling significantly. The Tc value of Ca2UH18-P-3m1 is
estimated to be 57.5-65.8 K at 100 GPa. Our studies demonstrate that
introducing actinides into alkaline-earth metal hydrides provides possibility
in designing novel superconducting ternary hydrides.
