Chemical control of hydrogen cage geometry and electron-phonon coupling in cubic NaXH₁₂ superconducting hydrides

Hydrogen-rich hydrides are promising candidates for high-temperature superconductivity under megabar pressures. In this work, we perform systematic first-principles calculations on cubic Pm3̄m ternary hydrides NaXH₁₂, which serve as a fixed hydrogen-framework platform to tune hydrogen cage geometry via central metal atom substitution. All compositions stabilize an intact H₂₄ cage via combined ionic metal-hydrogen interactions and covalent intra-cage H-H bonds, and their dynamical stability exhibits a systematic variation with the atomic radius of X. Larger central X atoms expand the H₂₄ framework and elongate average H-H bonds, which softens mid-frequency hydrogen vibrational modes and amplifies the overall electron-phonon coupling dominated by the hydrogen sublattice. By contrast, external pressure compresses the hydrogen cage, hardens hydrogen phonon branches and suppresses coupling strength. At 200 GPa, the superconducting transition temperatures of the relevant compounds, calculated via the Allen-Dynes modified McMillan equation, range from 199.14 to 225.30 K, with NaYH₁₂ reaching a maximum T_c of 225.30 K. This work establishes an effective chemical tuning route for hydrogen clathrate superconductors: atom radius engineering modulates H-H bonding, hydrogen phonon softening and superconducting performance, providing guidance for optimizing superconducting performance through chemical tuning of the hydrogen cage geometry.