Polymorphs of 1,1-diamino-2,2-dinitroethene (FOX-7): Isothermal compression versus isobaric heating

Raman spectroscopy was used to examine polymorphic changes in 1,1-diamino-2,2-dinitroethene (FOX-7) single crystals under: isothermal compression to 15 GPa and isobaric heating to 500 K. Changes in the Raman spectra were observed at ∼2.0 and ∼4.5 GPa, and at ∼390 K and ∼450 K. These onsets are in general accord with previously reported onsets from IR measurements under isothermal compression and from X-ray diffraction measurements under isobaric heating, respectively. In contrast to recent suggestions, we show that the high pressure polymorphs have different vibrational structures, and likely different crystal structures, than the high temperature polymorphs.