Alloy-Framework Hydrides with Embedded Quasi-Molecular H₂ units: A Promising Configuration for High-T_c Superconductors at Moderate Pressures

The exploration of high-T_c superconducting hydrides at ambient pressure is significant for the field of physics and materials science. Herein, by employing hydrogen-storage alloys as the template, we incorporate quasi-molecular H₂ units into binary Laves phase alloy Fd3m - A₂X, thereby achieving robust electron-phonon coupling at significantly reduced pressures. Through high-throughput screening of Fd3m-A₂XH₁₆ (A = group IIA elements, X = group IIIB elements), we identify eight dynamically stable compounds within 150 GPa. Among these candidates, Mg₂AcH₁₆ is dynamically stable at 20 GPa, and Ca₂AcH₁₆ maintains dynamic stability at ambient pressure. Our calculations demonstrate that charge transfer occurs from A and X atoms to the antibonding orbitals of elongated H₂ units (with H-H bond lengths from 0.85 to 0.96 Å), which concurrently activates mid-frequency H-dominated phonon modes that dominate electron-phonon coupling strength. Notably, Mg₂AcH₁₆ exhibits a superconducting transition temperature of 221 K at 70 GPa (λ = 5.17 and quality factor S = 4.34). Ca₂AcH₁₆ has a T_c of 165 K at 95 GPa. Our work advances the fundamental understanding of high temperature superconductors and provides a viable strategy for further discovery of high temperature superconductors at ambient pressure.