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_\rm 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 H_2 units into binary Laves phase alloy Fd\bar3m-A_2X, thereby achieving robust electron-phonon coupling (EPC) at significantly reduced pressures. Through high-throughput screening of Fd\bar3m-A_2X\rm H_16 (A = group IIA elements, X = group IIIB elements), we identify eight dynamically stable compounds within 150 GPa. Among these candidates, Mg_2AcH_16 is dynamically stable at 20 GPa, and Ca_2AcH_16 maintains dynamic stability at ambient pressure. Our calculations demonstrate that charge transfer occurs from A and X atoms to the antibonding orbitals of elongated H_2 units (with H-H bond lengths from 0.85 to 0.96 Å), which concurrently activates mid-frequency H-dominated phonon modes that dominate EPC strength. Notably, Mg_2AcH_16 exhibits a superconducting transition temperature of 221 K at 70 GPa (\lambda = 5.17 and quality factor S = 4.34). Ca_2AcH_16 has a T_\rm c of 165 K at 95 GPa. Our work advances the fundamental understanding of high-temperature superconductors and provides a viable strategy for the further discovery of high-temperature superconductors at ambient pressure.