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 (<i>T</i>_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 <i>C</i>2/<i>c</i> phase (600 GPa), the IAEs are connected to the S atomic extra-nuclear electrons with the unconventional <i>d</i> orbital attribute due to the extreme pressure, while two low-pressure <i>R</i>-3 (25 GPa) and <i>C</i>2/<i>m</i> (400 GPa) phases have interconnected IAEs. Due to its unique IAE attributes, <i>C</i>2/<i>c</i> Li₉S exhibits the highest <i>T</i>_c of 53.29 K at 600 GPa. Its superconductivity results from the coupling of the S <i>d</i>, Li <i>p</i> 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.