Effect of Polar Surfaces on Decomposition of Molecular Materials

We report polar instability in molecular materials. Polarization-induced explosive decomposition in molecular crystals is explored with an illustrative example of two crystalline polymorphs of HMX, an important energetic material. We establish that the presence of a polar surface in δ-HMX has fundamental implications for material stability and overall chemical behavior. A comparative quantum-chemical analysis of major decomposition mechanisms in polar δ-HMX and nonpolar β-HMX discovered a dramatic difference in dominating dissociation reactions, activation barriers, and reaction rates. The presence of charge on the polar δ-HMX surface alters chemical mechanisms and effectively triggers decomposition simultaneously through several channels with significantly reduced activation barriers. This results in much faster decomposition chemistry and in higher chemical reactivity of δ-HMX phase relatively to β-HMX phase. We predict decomposition mechanisms and their activation barriers in condensed δ-HMX phase, sensitivity of which happens to be comparable to primary explosives. We suggest that the observed trend among polymorphs is a manifestation of polar instability phenomena, and hence similar processes are likely to take place in all polar molecular crystals.