The experimental realization of high-temperature superconductivity in compressed hydrides ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{LaH}}_{10}$ under high pressures over 150 GPa has aroused great interest in reducing the stabilization pressure of superconducting hydrides. For cerium hydride ${\mathrm{CeH}}_9$ recently synthesized at $80–100$ GPa, our first-principles calculations reveal that the strongly hybridized electronic states of $\mathrm{Ce}\text{−}4f$ and $\mathrm{H}\text{−}1s$ 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 symmetry-driven van Hove singularity to increase the density of states, which in turn significantly raises a superconducting transition temperature $T_c$. We show that hole doping with ${\mathrm{Ce}}^{3+}$ ions can be very electronically miscible in ${\mathrm{CeH}}_9$ because both ${\mathrm{Ce}}^{3+}$ and Ce behave similarly as cations. Therefore, the interplay of crystalline symmetry, band topology, and hole doping contributes to enhance $T_c$ in compressed ${\mathrm{CeH}}_9$, which can also be demonstrated in another superconducting rare-earth hydride, ${\mathrm{LaH}}_{10}$.

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• Dopants
• Electronic structure
• Impurities in superconductors
• Pressure effects
• Superconducting phase transition
• High-temperature superconductors
• Hydrides
• Density functional calculations
• Methods in superconductivity