Helium-nitrogen mixtures 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. Among these high-pressure compounds, the monoclinic structure of ${\mathrm{HeN}}_{22}$ consists of neutral He atoms, partially ionic dimers ${\mathrm{N}}_{2}^{\ensuremath{\delta}\ensuremath{-}}$, and lanternlike cages ${\mathrm{N}}_{20}^{\ensuremath{\delta}+}$, showing a certain amount of charge transfer within the nitrogen framework. The monoclinic (${\mathrm{HeN}}_{20}{)}^{\ensuremath{\delta}+}{\mathrm{N}}_{2}^{\ensuremath{\delta}\ensuremath{-}}$ may be quenchable to ambient pressure with the estimated energy density of 10.44 kJ/g, which is $\ensuremath{\sim}2.4$ times larger than that of trinitrotoluene, indicating a very promising high-energy-density material.