New Route to High-Energy Density Polymeric Nitrogen “t-N” via He-N Compounds

Polymeric forms of nitrogen, stabilized by compressing pure molecular nitrogen, have not yet been recovered to ambient conditions, precluding their practical application as high-energy density materials. Here, we highlight a possible route to the formation of a novel tetragonal polymeric nitrogen via He-N compounds at high pressures. By combining first-principles calculations with structure searching, we predict the existence of a new class of nitrogen-rich nitrides with a stoichiometry of HeN₄ that are energetically stable (relative to a mixture of solid He and N₂) above 8.5 GPa. HeN₄ adopts a structure consisting of a polymeric channel-like nitrogen framework filled with linearly-arranged helium atoms at pressures above 95 GPa. The nitrogen framework persists to ambient pressure on decompression after the removal of helium atoms, forming a new pure polymeric nitrogen denoted as t-N. t-N is dynamically and mechanically stable at ambient pressure and has an estimated energy density of ~11.31 kJ/g, which marks it out as a remarkable high-energy density material. This expands the known polymeric forms of nitrogen and provides a new potential route to synthesise it.