Probing Crystal Growth of ε- and α‑CL-20 Polymorphs via Metastable Phase Transition Using Microscopy and Vibrational Spectroscopy

This article presents evidence of a possible route to the formation of ε- and α-polymorphic phases of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) studied through CL-20 solution using a reverse and normal precipitation method. Reverse precipitation with instant addition facilitated the opportunity to track the crystal phases from their immediate formation to the end of phase stabilization. Precipitation under apparent conditions to achieve α- or ε-phases showed the initial occurrence of a metastable β-phase and subsequent transformation to the intended stable phases. The β-phase showed sufficiently longer stability while under specified conditions for ε- than in a hydrated medium set to obtain the α-phase. Transformation of fine needle-shaped β-CL-20 crystals to uniform diamond-shaped α- or bipyramidal ε-habit had been observed to pass through an equilibrium state of dissolution and reprecipitation. This work also elaborates the effect of crystallization methodology on conversion time. Vibrational spectroscopy and microscopic techniques were employed to track time-dependent polymorphic conversions. A drastic reduction in β → ε conversion time, from 160 to 10 min, could have been affected by using ultradispersed seed crystals. We thus also demonstrated a hazard free nongrinding method to prepare ε-CL-20 with particle size < 10 μm through precipitation and their effect on thermal stability and mechanical sensitivity.