Hydrogen-based compounds are promising candidates for room-temperature superconductivity. However, hydrogen-related anharmonic quantum effects have created a huge gap between experiments and theories. The compound $\mathrm{Rb}{\mathrm{H}}_6$ exemplifies the impacts of quantum fluctuations and lattice anharmonicity on crystal predictions based on the classical Born-Oppenheimer approximation. The zero-point energy triggers a thermodynamic rearrangement, transforming the originally harmonic metastable $\mathrm{Y}{\mathrm{B}}_6$-type structure into the ground-state quantum phase of stoichiometric rubidium hexahydride at ambient pressure. During the quantum variational energy minimization, atomic displacements are dictated by the competition of bond energies, with anharmonicity favoring the preservation of strong chemical bonds at the expense of weaker ones. Quantum anharmonicity induces the contraction of electronic bands near the Fermi level, exacerbating Peierls instability and enhancing electron-phonon coupling matrix elements. Furthermore, the strengthened electron-phonon coupling is attributed to anharmonicity-induced phonon softening and intensified partial Fermi surface nesting. Consequently, $\mathrm{Y}{\mathrm{B}}_6$-type $\mathrm{Rb}{\mathrm{H}}_6$ exhibits the record superconducting transition temperature $T_c$ of 180 K at ambient pressure, surpassing both the theoretical prediction for $\mathrm{M}{\mathrm{g}}_2\mathrm{Ir}{\mathrm{H}}_6$ of 160 K and the experimental record held by $\mathrm{HgB}{\mathrm{a}}_2\mathrm{C}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_8$ of 134 K. It reveals the potential of phonon renormalization for even higher $T_c$, opening new avenues for the exploration of ambient superconductivity.

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• Electronic structure
• High-pressure studies
• High-temperature superconductors
• Hydrides
• Superconductors
• BCS theory
• Density functional calculations
• First-principles calculations