Designing Ambient Pressure Superconductivity in Li-Mg-Based Hydrides via Transition-Metal Modulation: From Quaternary to Quinary Systems.

Hydrogen-rich superconductors have emerged as promising candidates for achieving high-temperature superconductivity, yet their practical applications are generally limited by the requirement for high external pressures. In this work, taking the Fm-3-XYZ2H12 structure as a prototype, we constructed a series of LiMgM2H12 quaternary hydrides and the LiMgZrHfH12 quinary hydride. High-throughput screening indicates that the three hydrides, LiMgZr2H12, LiMgHf2H12, and LiMgZrHfH12, are dynamically stable at ambient pressure but thermodynamically metastable. Electron-phonon coupling calculations show that the Tc values of LiMgZr2H12, LiMgHf2H12, and LiMgZrHfH12 reach 87.4, 81.2, and 85.2 K at ambient pressure, respectively, all exceeding the boiling point of liquid nitrogen and demonstrating excellent superconducting properties. Further analysis reveals that, compared with the Ga-based parent Fm-3-XYZ2H12 hydrides, Li substitution markedly reconstructs the electronic-state distribution near the Fermi level and enhances the contribution of H atoms to the density of states near the Fermi level. This promotes the coupling between conducting electrons and high-frequency hydrogen vibrations. Such strong electron-phonon coupling plays a crucial role in their high-temperature superconductivity. These findings provide valuable insights into the theoretical design of high-Tc superconductors under ambient pressure and offer theoretical guidance for future experimental studies in this field.