Polymorphism and Polymorphic Transition in Energetic Molecular Crystals

Polymorphism is a universal phenomenon of both natural and artificial substances, and refers to that a substance can exist in different crystalline forms. Correspondingly, that one form changes to another is polymorphic transition. It occurs as one form is thermodynamically unstable and kinetically permitted to be transferred to another. Thereafter, molecular and crystal structures, property and performance change more or less. Generally, there is no chemical reaction included during a polymorphic transition. Energetic compounds, usually organic molecules composed of C, H, N, and O atoms, are inherently polymorphic thanks to the variations of molecular conformation and molecular stacking. The polymorphic transition of energetic materials may occur under allowed conditions like heating and pressing, during their lifetime from design & synthesis to crystallization, powder manufacture, machining, transportation, storage, and application. Polymorphic transition is often accompanied by varied structures, properties, and performances, and is a common phenomenon of energetic materials.