  The energy landscape of helium-nitrogen mixtures is explored by ab initio
evolutionary searches, which predicted several stable helium-nitrogen compounds
in the pressure range from 25 to 100 GPa. In particular, the monoclinic
structure of HeN$_{22}$ consists of neutral He atoms, partially ionic dimers
N$_{2}$$^{\delta-}$, and lantern-like cages N$_{20}$$^{\delta+}$. The presence
of helium not only greatly enhances structural diversity of nitrogen solids,
but also tremendously lowers the formation pressure of nitrogen salt. The
unique nitrogen framework of (HeN$_{20}$)$^{\delta+}$N$_{2}$$^{\delta-}$ may be
quenchable to ambient pressure even after removing helium. The estimated energy
density of N$_{20}$$^{\delta+}$N$_{2}$$^{\delta-}$ (10.44 kJ/g) is $\sim$2.4
times larger than that of trinitrotoluene (TNT), indicating a very promising
high-energy-density material.
