Helium-nitrogen mixtures at high pressure

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