Cocrystals of 1,3,5,7-Tetranitro-1,3,5,7-tetrazacyclooctane (HMX)

Energetic materials (energetics), including explosives, propellants, and pyrotechnics, are an important class of materials for which performance depends critically on solid-state properties such as density and stability. Cocrystallization is emerging as a strategy to enhance materials properties of new and existing energetics; however, few examples exist where the principles to enable the successful engineering of such cocrystals have been delineated. To address this need, the cocrystallization of 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), a powerful and widely used explosive, was investigated. Seven cocrystals of HMX were structurally elucidated using single crystal X-ray diffraction. These cocrystal structures, in addition to three previously reported structures, reveal at least three distinct structural motifs by which HMX cocrystals form, each dictated by the electronic character and size of the cocrystal former. From these structures, general trends emerge having direct implications for cocrystal properties including density, packing coefficient, sensitivity to impact, and thermal behavior. In addition, these impact-insensitive energetic cocrystals can be restored to sensitive delta-HMX upon gentle heating, demonstrating the potential for these cocrystals to function as smart explosive materials.