  Helium (He) is the most inert noble gas at ambient conditions. It adopts a
hexagonal close packed structure (P63/mmc) and remains in the insulating phase
up to 32 TPa. In contrast, lithium (Li) is one of the most reactive metals at
zero pressure, while its cubic high-pressure phase (Fd-3m) is a weak metallic
electride above 475 GPa. Strikingly, a stable compound of Li5He2 (R-3m) was
formed by mixing Fd-3m Li with P63/mmc He above 700 GPa. The presence of helium
promotes the lattice transformation from Fd-3m Li to Pm-3m Li, and tuns the
three-dimensional distributed interstitial electrons into the mixture of zero-
and two-dimensional anionic electrons. This significantly increases the degree
of metallization at the Fermi level, consequently, the coupling of conductive
anionic electrons with the Li-dominated vibrations is the key factor to the
formation of superconducting electride Li5He2 with a transition temperature up
to 26 K, dynamically stable to pressures down to 210 GPa.
