Polymorphism in hexanitrohexaazaisowurtzitane crystallized from solution

Abstract An investigation has been performed on the effect of the properties of solvent and anti-solvent, addition method of anti-solvent and crystallization temperature on the polymorphic transformation of HNIW precipitated from solution. The crystallization behavior of HNIW polymorphs were confirmed by powder X-ray diffraction (PXRD) and the polymorphic content was evaluated by Rietveld analysis. It was observed that the dipole moment of anti-solvent and the supersaturation were related to the variations in polymorphic form. The metastable form β always was precipitated initially in high polarity anti-solvent or in high supersaturation case, and then transformed to the stable form e by solution mediated process, which obeyed the Ostwald's rule of stages. A new acetone solvate was isolated in acetone solution, which was so unstable that it quickly desolated to obtain form β. The stability of the hydrate form α depends on the degree of hydration and hence the low hydration degree of form α precipitated from solution can transform to form e. Forms γ and e are enantiotropic relationship and the transformation temperature T c is associated with the solvent system. The investigation of the polymorphic transformation of HNIW contributes to the understanding of the crystallization mechanism and the preparing of the four pure forms of HNIW.