Superconducting Electride Li9S with a Transition Temperature above the McMillan Limit

An electride, characterized by unique interstitial anionic electrons (IAEs), offers promising avenues for modulating its superconductivity. The pressure-dependent coupling between IAEs and orbital electrons significantly affects the superconducting transition temperature (T c). However, existing research has predominantly concentrated on pressures within 300 GPa. To advance the understanding, we propose investigating the Li–S system under ultrahigh pressure to unveil novel electride superconductors. Five stable Li-rich electrides with diverse IAE topologies, including one Li7S, three Li9S, and one Li12S phases, are identified through structural search calculations. Among the Li9S phases, in the C2/c phase (600 GPa), the IAEs are connected to the S atomic extra-nuclear electrons with the unconventional d orbital attribute due to the extreme pressure, while two low-pressure R-3 (25 GPa) and C2/m (400 GPa) phases have interconnected IAEs. Due to its unique IAE attributes, C2/c Li9S exhibits the highest T c of 53.29 K at 600 GPa. Its superconductivity results from the coupling of the S d, Li p electrons, and IAEs with the low-frequency phonons associated with the attraction between IAEs and the Li–S framework. Our work enhances insights into IAEs within electrides and their role in facilitating superconductivity.