Helium Induced Nitrogen Salt at High Pressure

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.