Stability of NNO and NPO Nanotube Crystals.

We combine the USPEX evolution searching method with density functional theory using dispersion corrections (DFT-ulg) to predict the crystal structure of the NNO extended solid at high pressures (from 100 to 500 GPa). We find that the NNO nanotube (with diameter ≈ 2.5 Å) is the most stable form above 180 GPa. We report here the stability, electronic properties, and mechanical properties of this novel nanotube and show that it is stable above 20 GPa. To find a similar structure that might be stable at ambient conditions, we considered the NPO tube and show that it is stable at zero pressure. The NPO phase leads to an insulator to metal transition at 25 GPa, where the PP van der Waals distance approaches the covalent bond distance. The energy content of this NPO nanotube crystal is 10.6 kJ/g, which is 152% higher than that of TNT and 86% higher than that of the HMX energetic material. This is the first example of a structural energetic material, which could have important applications in igniters, incendiaries, screening smoke ammunition, and similar devices. This process illustrates how materials discovery in extreme conditions can be used to discover and stabilize novel structures.