Superconducting Electride Li₉S with a Transition Temperature above the McMillan Limit

Abstract

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 Li 7 S, three Li 9 S, and one Li 12 S phases, are identified through structural search calculations. Among the Li 9 S 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 lowpressure R-3 (25 GPa) and C2/m (400 GPa) phases have interconnected IAEs. Due to its unique IAE attributes, C2/c Li 9 S 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.