The outlook for platonic and cubic gauche nitrogens

Abstract

While polynitrogens are proposed to be the hyper rocket fuels, many of their properties in the solid states are unclear. Herein, we comprehensively investigated the thermodynamic and kinetic stabilities, polymorphs, and rocket performances of platonic and cubic gauche nitrogens, i.e. tetrahedral N4, cubic N8, platonic dodecahedral N20 and cubic gauche nitrogen (cg-N), using the first-principles calculations. The results showed that the dissociation barrier of tetrahedral N4 in the gas phase (>200 kJ mol−1) is high enough to keep stable. It would be a liquid monopropellant under slight external pressure or low temperature. The specific impulse (Isp) of tetrahedral N4 is comparable to that of liquid H2/O2, but it provides a density of 1.459 g cm−3 with the body centered tetragonal packing. The conversion barrier of cubic N8 in the gas phase (∼85 kJ mol−1) is marginal for stability. It may be a solid at ambient temperature, and its Isp exceeds that of liquid H2/O2. The crystal density of cubic N8 is 1.964 g cm−3 with the rhombohedral centered hexagonal packing. The dissociation barrier for stability of platonic N20 in the gas phase (∼25 kJ mol−1) is insufficient, however it possesses a lattice energy above 100 kJ mol−1. Although the Isp of platonic N20 is as powerful as that of the liquid H2/O2, it has a crystal density of 2.132 g cm−3 with the face centered cubic packing. For surface relaxation, cg-N is unstable at the ambient condition with a dissociation barrier of ∼17 kJ mol−1. While the Isp of cg-N is less than those of platonic nitrogens, its effective impulse (Ief) is the best due to an extremely high density of 3.401 g cm−3. In respect of the thermodynamics, the enthalpies per atom of platonic nitrogens are always higher than those of cg-N and ε-N2 from 0 to 100 GPa.