Metal-Free Molecular Perovskite High-Energetic Materials

Metal-free energetic materials generally have the advantages of high gas yield and metal-free residue after combustion or explosion, enabling them to be widely used as explosives and propellant components. As part of a series of our investigations on ABX₃ molecular perovskite high-energetic materials, here we report five new metal-free members, (H₂A)­[NH₄(ClO₄)₃], by using different organic cations H₂A²⁺, i.e., 1-hydroxy-1,4-diazabicyclo[2.2.2]­octane-1,4-diium for DAP-O4, piperazine-1,4-diium for PAP-4, 1-methyl-piperazine-1,4-diium for PAP-M4, homopiperazine-1,4-diium for PAP-H4, and 1-methyl-1,4-diazabicyclo-[2.2.2]­octane-1,4-diium for DAP-M4, respectively. Together with the previously reported member, (H₂dabco)­[NH₄(ClO₄)₃] (DAP-4, H₂dabco²⁺ = 1,4-diazabicyclo[2.2.2]­octane-1,4-diium), these six metal-free molecular perovskite high-energetic materials provide nice instances to fine-tune the oxygen balance, crystal density, thermal stability, and detonation performance, by changing the A-site organic cations solely. The density functional theory (DFT) calculations and the Kamlet–Jacob (K–J) equation suggested that improving the oxygen balance while keeping the spherical shape of the organic cations to match the anionic cage in these metal-free energetic materials facilitates obtaining a better detonation performance, providing an important clue for designing advanced practicable high-energetic materials. It is worth noting that three compounds (PAP-4, PAP-H4, and DAP-O4) are expected to exceed the performances of RDX as both explosive and propellant, in which DAP-O4 has the highest detonation heat (6.21 kJ mol^–1), detonation velocity (8.900 km s^–1), and detonation pressure (35.7 GPa), as well as a higher specific impulse value (262 s).