Pentazole and Ammonium Pentazolate: Crystalline Hydro-Nitrogens at High Pressure

Two new crystalline compounds, pentazole (N₅H) and ammonium pentazolate (NH₄)­(N₅), both featuring cyclo-N₅^– are discovered using a first-principles evolutionary search of the nitrogen-rich portion of the hydro-nitrogen binary phase diagram (N_xH_y, x ≥ y) at high pressures. Both crystals consist of the pentazolate N₅^– anion and ammonium NH₄⁺ or hydrogen H⁺ cations. These two crystals are predicted to be thermodynamically stable at pressures above 30 GPa for (NH₄)­(N₅) and 50 GPa for pentazole N₅H. The chemical transformation of ammonium azide (NH₄)­(N₃) mixed with dinitrogen (N₂) to ammonium pentazolate (NH₄)­(N₅) is predicted to become energetically favorable above 12.5 GPa. To assist in identification of newly synthesized compounds in future experiments, the Raman spectra of both crystals are calculated and mode assignments are made as a function of pressure up to 75 GPa.