High Pressure-Temperature Phase Diagram of 1,1-diamino-2,2-dinitroethylene*

1,1-diamino-2,2-dinitroethelyne (FOX-7) is a less sensitive 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 $, \quad $\beta $, and $\gamma )$. In this study, we have investigated the high P-T stability of FOX-7 polymorphs using synchrotron mid-infrared (MIR) spectroscopy. At ambient pressure, our MIR spectra confirmed the known $\alpha \quad \to \quad \beta $ (110 $^{\circ}$C) and $\beta \quad \to \quad \gamma $ (160 $^{\circ}$C) phase transitions; as well as, indicated an additional phase transition, $\gamma \quad \to \quad \delta $ (210$^{\circ}$C), with the $\delta $ phase being stable up to 250 $^{\circ}$C prior to melt/decomposition. In situ MIR spectra obtained during isobaric heating at 0.9 GPa revealed that the $\alpha \quad \to \quad \beta $ transition occurs at 180$^{\circ}$C, while $\beta $ $\to \quad \beta +\delta $ phase transition shifted to 300$^{\circ}$C with suppression of $\gamma $ phase. Decomposition was observed above 325$^{\circ}$C. Based on multiple high P-T measurements, we have established the first high P-T phase diagram of FOX-7.