High pressure-temperature polymorphism of 1,1-diamino-2,2-dinitroethylene

1,1-diamino-2,2-dinitroethylene (FOX-7) is a low sensitivity energetic material with performance comparable to commonly used secondary explosives such as RDX and HMX. At ambient pressure, FOX-7 exhibits complex polymorphism with at least three structurally distinct phases (alpha, beta, and gamma). In this study, we have investigated the high pressure-temperature stability of FOX-7 polymorphs using synchrotron mid-infrared (MIR) spectroscopy. At ambient pressure, our MIR spectra and corresponding differential scanning calorimetry (DSC) measurements confirmed the known alpha --> beta (similar to 110 degrees C) and beta --> gamma (similar to 160 degrees C) structural phase transitions; as well as, indicated an additional transition gamma --> delta (similar to 210 degrees C), with the delta phase being stable up to similar to 251 degrees C prior to decomposition. In situ MIR spectra obtained during isobaric heating at 0.9 GPa, revealed a potential alpha --> beta transition that could occur as early as 180 degrees C, while beta --> beta+delta phase transition shifted to similar to 300 degrees C with suppression of gamma phase. Decomposition was observed slightly above 325 degrees C at 0.9 GPa.