  Helium is generally understood to be chemically inert and this is due to its
extremely stable closed-shell electronic configuration, zero electron affinity
and an unsurpassed ionization potential. It is not known to form
thermodynamically stable compounds, except a few inclusion compounds. Here,
using the ab initio evolutionary algorithm USPEX and subsequent high-pressure
synthesis in a diamond anvil cell, we report the discovery of a
thermodynamically stable compound of helium and sodium, Na2He, which has a
fluorite-type structure and is stable at pressures >113 GPa. We show that the
presence of He atoms causes strong electron localization and makes this
material insulating. This phase is an electride, with electron pairs localized
in interstices, forming eight-centre two-electron bonds within empty Na8 cubes.
We also predict the existence of Na2HeO with a similar structure at pressures
above 15 GPa.
