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 ${\mathrm{N}}_2^{\delta -}$, and lanternlike cages ${\mathrm{N}}_{20}^{\delta +}$, showing a certain amount of charge transfer within the nitrogen framework. The monoclinic (${\mathrm{HeN}}_{20}{)}^{\delta +}{\mathrm{N}}_2^{\delta -}$ may be quenchable to ambient pressure with the estimated energy density of 10.44 kJ/g, which is $\sim 2.4$ times larger than that of trinitrotoluene, indicating a very promising high-energy-density material.

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• Charge
• Chemical bonding
• First-principles calculations
• Phase transitions
• Pressure effects