  The experimental realization of high-temperature superconductivity in
compressed hydrides H$_3$S and LaH$_{10}$ at high pressures over 150 GPa has
aroused great interest in reducing the stabilization pressure of
superconducting hydrides. For cerium hydride CeH$_9$ recently synthesized at
80$-$100 GPa, our first-principles calculations reveal that the strongly
hybridized electronic states of Ce 4$f$ and H 1$s$ orbitals produce the
topologically nontrivial Dirac nodal lines around the Fermi energy $E_F$, which
are protected by crystalline symmetries. By hole doping, $E_F$ shifts down
toward the topology-driven van Hove singularity to significantly increase the
density of states, which in turn raises a superconducting transition
temperature $T_c$ from 74 K up to 136 K at 100 GPa. The hole-doping
concentration can be controlled by the incorporation of Ce$^{3+}$ ions with
varying their percentages, which can be well electronically miscible with Ce
atoms in the CeH$_9$ matrix because both Ce$^{3+}$ and Ce behave similarly as
cations. Therefore, the interplay of symmetry, band topology, and hole doping
contributes to enhance $T_c$ in compressed CeH$_9$. This mechanism to enhance
$T_c$ can also be applicable to another superconducting rare earth hydride
LaH$_{10}$.
