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 HeN22 consists of neutral He atoms, partially ionic dimers N-2(delta-), and lanternlike cages N-20(delta-), showing a certain amount of charge transfer within the nitrogen framework. The monoclinic (HeN20)N-delta+(2)delta- may be quenchable to ambient pressure with the estimated energy density of 10.44 kJ/g, which is similar to 2.4 times larger than that of trinitrotoluene, indicating a very promising high-energy-density material.