The discovery of ${\mathrm{LaSc}}_2{\mathrm{H}}_{24}$ represents an advance in the quest for room-temperature superconductivity, yet the microscopic mechanism underlying its high-temperature superconductivity remains unclear. Through a comprehensive revisit of theoretical analysis, we uncover a pivotal transition from the anisotropic two-gap superconductivity of ${\mathrm{LaH}}_{10}$ to an isotropic single-gap superconductivity in ${\mathrm{LaSc}}_2{\mathrm{H}}_{24}$ upon the introduction of scandium, thereby enhancing the superconducting critical temperature $\left(T_{\mathrm{c}}\right)$. This enhancement is rooted in a critical dual role of Sc $3d$ electrons: (i) the Sc-derived Jahn-Teller effect promotes hydrogen metallization via the elongation of specific interlayer H-H bonds and enhances electron-phonon coupling (EPC) through the softening of associated phonon modes; (ii) Sc $3d$ electrons reconstruct the electronic structure into an ${\mathrm{MgB}}_2$-like configuration, generating Sc-H-Sc $\sigma$- and $\delta$-bonding states with EPC strengths comparable to ${\mathrm{LaH}}_{10}$. Crucially, the pronounced orbital overlap between Sc and the hydrogen cages effectively unifies these two contributions on the Fermi surface. This Sc-induced gap unification bridges the high-EPC H-H states with widespread Sc-H states, establishing an isotropic single-gap nature with a large overall EPC strength. Our findings identify this Sc-induced gap unification as the fundamental mechanism for achieving room-temperature superconductivity in ${\mathrm{LaSc}}_2{\mathrm{H}}_{24}$, offering a theoretical blueprint for the future design of superior superconducting hydrides.

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• Chemical bonding
• Electron-phonon coupling
• Impurities in superconductors
• Pressure effects
• Superconducting gap
• Superconducting phase transition
• High-temperature superconductors
• Hydrides
• Density functional theory
• First-principles calculations