Electrides are unique compounds where most of the electrons reside at interstitial regions of the crystal behaving as anions, which strongly determines its physical properties. Interestingly, the magnitude and distribution of interstitial electrons can be effectively modified either by modulating its chemical composition or external conditions (e.g. pressure). Most of the electrides under high pressure are non-metallic, and superconducting electrides are very rare. In this work we report that a pressure-induced stable Li₆P electride, identified by first-principles swarm structure calculations, becomes a superconductor with a predicted superconducting transition temperature T_c of 39.3 K, which is the highest among the already known electrides [1]. The interstitial electrons in Li₆P, with dumbbell-like connected electride states, play a dominant role in the superconducting transition. Other Li-rich phosphides, Li₅P, Li₁₁P₂, Li₁₅P₂ and Li₈P, are also predicted to be superconducting electrides, but with a lower T_c. Superconductivity in all these compounds can be attributed to a combination of a weak electronegativity of P with a strong electropositivity of Li, and opens up the interest to explore high-temperature superconductivity in similar binary compounds.

[1] Z. Zhao, S. Zhang, T. Yu, H. Xu, A. Bergara and G. Yang; Phys. Rev. Lett. 122, 097002 (2019).