Solvent Effects on the Growth Morphology and Phase Purity of CL-20

Abstract

The performance and stability of the high energy secondary explosive 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW, also known as CL-20) can be affected by factors including the phase purity of the bulk material, as well as the particle size, morphology, and defect density of the individual crystallites. Slow evaporation crystallization of CL-20 from 16 different single solvent and co-solvent systems was performed. The phase purity of the bulk material obtained was analyzed by powder X-ray diffraction, optical microscopy, and differential scanning calorimetry. These complementary methods confirmed that under most of the slow evaporation conditions examined, a concomitant mixture of two or more crystalline phases was usually obtained. Numerous individual crystal morphologies were determined using single crystal X-ray goniometry and compared against calculated BFDH morphologies. Examination of the packing interactions in the different CL-20 phases via Hirshfeld surface analysis provides some insight into why concomitant polymorphism is so frequently observed.