Pushing the limits of oxygen balance in pentazolate salts via the theoretical construction of oxidizer-based cocrystal

Abstract

Designing high-energy-density materials (HEDMs) based on nonmetallic cyclo-pentazolate (c-N₅⁻) salts remains challenging because of their low intrinsic density and unfavorable oxygen balance. Here, we introduce a zero-oxygen-balance (based on CO) cocrystal strategy, pairing c-N₅⁻ salts with energetic oxidizers such as ammonium dinitramide (ADN) and hydrazinium nitroformate (HNF). Using a constrained evolutionary search, two stable cocrystals, [NH₄][N₅]·ADN and [N₂H₅][N₅]·HNF, are predicted. [NH₄][N₅]·ADN is predicted to be most stable in a monoclinic P2₁/c setting, whereas [N₂H₅][N₅]·HNF favors an orthorhombic Pna2₁ arrangement; both are reinforced by extensive hydrogen-bond networks. Electronic structure calculations show reduced band gaps relative to the parent salts, indicating increased electronic activity and identifying N–N and N–O bonds as the most labile sites. Ab initio molecular dynamics indicate thermal robustness of the cocrystals on the simulated timescale (up to 800 K). Energetic performance estimated under the same computational protocol yields predicted detonation velocities of 9.61 and 9.82 km s⁻¹ and pressures of 39.55 and 42.49 GPa for [NH₄][N₅]·ADN and [N₂H₅][N₅]·HNF, respectively; the detonation velocity values exceed those predicted for the parent salts and for benchmark explosives such as β-CL-20. These results suggest that oxidizer integration via cocrystallization offers a practical route to mitigate the intrinsic limitations of c-N₅⁻ salts and to guide design of next-generation HEDMs.