  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 becomes superconductor with a Tc of 39.3
K, which is the highest among already known electrides. The interstitial
electrons in Li6P, with dumbbell-like connected electride states, play a
dominant role in the superconducting transition. Other Li-rich phosphides, Li5P
and Li8P, are also predicted to be superconducting electrides, but with a lower
Tc. 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.
