Prediction of enhanced superconductivity in cyclo- H 12 Bi / Pb involving a resonant hydrogen structure

Hydrogen-based compounds demonstrate high-temperature superconductivity under pressure, offering promising avenues for exploring metal hydrides with elevated critical temperatures ( T c ), while hydrides comprising H 2 molecular units were previously deemed unfavorable. Searching for new forms of hydrogen within metal hydrides promised a route for developing novel high-temperature superconductors. In this study, we utilized the evolutionary algorithm alongside first-principles calculations to investigate the high-pressure crystal structure of the Bi H n ( n = 7 − 18 ) system. Notably, we discovered stable cyclo- H 12 Bi / Pb compounds in which the H 12 resembles cyclohexanelike cyclo- H 12 rings. These compounds are stable above 180 GPa and exhibit a higher T c than Bi H 8 composed of H 2 units. In cyclo- H 12 , the hydrogen has a slightly elongated intramolecular H-H length compared to Bi H 8 , while the intermolecular H-H distance is much shorter than in H cages, forming a resonant structure. The resonance led to in-/out-of-plane vibration modes for the six H 2 units in cyclo- H 12 , forming broad midfrequency vibrational bands and boosting the electron-phonon coupling. These findings highlight the cycloform of hydrogen-based hydrides as promising candidates for high T c superconductors, propelling further exploration of superconductivity in binary superhydrides with different forms of hydrogen.