A high-pressure far- and mid-infrared study of 1,1-diamino-2,2-dinitroethylene.

High pressure infrared (IR) measurements of the insensitive explosive, 1,1-diamino-2,2-dinitroethylene (DADNE or FOX-7), have been performed in the far- and mid-IR spectral regions up to ∼28 GPa using a synchrotron source. Petroleum jelly and KBr were employed as the pressurizing media for the far- and mid-IR experiments, respectively. In both experiments, IR spectra were collected at various pressures both in compression and decompression to determine reversibility under pressure cycling. There is evidence for at least two and possibly three phase transitions in the samples at pressures near 2, 5, and10 GPa, respectively. In the high frequency region (∼3000-3300 cm-1), the NH2 symmetric and antisymmetric vibrational modes soften with increasing pressure, suggesting strengthening intermolecular hydrogen bonding, and then stiffen with further pressure increase. At higher pressures (above 5 GPa), we suspect progressive flattening of the zig zag structure characteristic of the α phase occurs, which was observed in the low temperature, quenched γ phase. FOX-7 was also observed to survive pressure cycling up to 28 GPa contrary to earlier reports of pressure-induced molecular decomposition. [ABSTRACT FROM AUTHOR]