Superconducting Transition Temperature in A 2 H Tuned by Electron-Phonon Parameters Competition with and without Interstitial Electrons

Based on first-principles calculations, we systematically investigate the charge distribution, electronic structure, and superconducting properties of A 2 H hydrides (A = Li, Na, K, Sc, Y, La, Lu, Cu, Ag). By combining Bader charge, topological analysis, electron localization function and partial charge density, we confirm that the spatial distribution of charge carriers varies significantly among different systems. Rare-earth hydrides exhibit locally confined electrons at lattice interstices. Noble metal hydrides have carriers concentrated in the metal-hydrogen bonding regions. Alkali metal hydrides show no effective interstitial carriers. Electron-phonon coupling results reveal that, in rare-earth systems, interstitial electrons preferentially couple with low-frequency metal vibrations, reducing the logarithmic average phonon frequency (ω log ). However, the d orbitals of rare-earth elements effectively enhance the density of states at the Fermi level. Together with multi-channel Fermi surfaces, this gives La 2 H the largest electron-phonon coupling strength (λ = 0.951) among all systems, partially offsetting the suppression of superconductivity caused by ω log . In noble metal hydrides, which lack interstitial electrons, high-frequency hydrogen vibrations dominate the coupling, leading to optimal superconducting performance. For example, Ag 2 H achieves a superconducting transition temperature of 24.8 K. In contrast, alkali metal hydrides exhibit very weak coupling and no obvious superconducting response. Our study demonstrates that the superconducting performance of A 2 H hydrides is determined by the synergistic competition between λ and ω log . The spatial distribution of charge carriers is a key factor in tuning their superconducting properties.