New Insights into the Metastable β Form of RDX

The secondary explosive RDX (1,3,5-trinitrohexahydro-1,3,5-triazine) has five known polymorphic forms, though only the α and β phases are observable at room temperature and pressure. Many literature reports suggest that the metastable β-RDX polymorph is extremely rare due to the limited number of solvents from which it can be grown and its facile conversion to the more stable α-RDX form. Herein we show that β-RDX can be consistently obtained using drop cast crystallization methods from a broad range of solvents [acetone, tetrahydrofuran (THF), nitromethane, dimethylsufoxide (DMSO)] and that crystals grown this way remain stable over extended periods of time up to a year. The distribution of α:β crystals as a function of solvent and drop concentration on various modified and unmodified glass surfaces was determined using Raman microscopy. β-RDX was the most prevalent phase deposited from dilute drops regardless of the solvent, though more concentrated DMSO drops tended to give either β or α exclusively. In cases where concomitant growth was observed, β crystals were typically smaller than α-RDX crystals. The morphological and thermal properties of β-RDX crystals were assessed, allowing for the most thorough characterization of the physical–chemical properties of this metastable phase to date. Mechanistic differences in the decomposition pathway of α-RDX and β-RDX single crystals upon exposure to electromagnetic radiation suggest some interesting intrinsic differences in their material properties.