First Principles Investigation of Nitrogen-Rich Energetic Materials

Energetic materials rich in nitrogen hold great promise due to their high positive heats of formation and because the products of detonation consist mostly of environmentally-friendly N$_{\mathrm{2}}$ gas. Pure polymeric nitrogen can be synthesized at high pressures, although it has yet to be quenched to ambient conditions. By introducing a reducing agent into the crystal structure, the stability of polynitrogen compounds can be enhanced. We have investigated the stability of alkali polynitrides (R$_{\mathrm{x}}$N$_{\mathrm{y}})$, as well as hydro-nitrogens (H$_{\mathrm{x}}$N$_{\mathrm{y}})$, at high pressures and found several stable crystal structures that contain N$_{\mathrm{5}}$ rings, N$_{\mathrm{6}}$ rings, N$_{\mathrm{4}}$ chains, and polymeric nitrogen chains. Using our theoretical input, cesium pentazolate salt has been synthesized for the first time by our experimental collaborators. We also consider several other ternary high-nitrogen energetic materials, containing \textbraceleft C, O, N\textbraceright and \textbraceleft H, O, N\textbraceright atoms. The results demonstrate the great potential for first principles structure prediction to design new energetic compounds and crystal structures, which are better, safer, and greener than traditional CHNO energetic materials.