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Updated: Apr 28, 2026

Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems
Published on: April 28, 2016
High-pressure vibrational and polymorphic response of 1,1-diamino-2,2-dinitroethene single crystals: Raman
Zbigniew A Dreger1, Yuchuan Tao, Yogendra M Gupta
1Institute for Shock Physics and Department of Physics and Astronomy, Washington State University , Pullman, Washington 99164-2816, United States.
Abstract:
Raman spectroscopy was used to examine the vibrational and polymorphic behavior of 1,1-diamino-2,2-dinitroethene (FOX-7) to elucidate its structural and chemical stability under high pressure. Measurements were performed on single crystals compressed in a diamond anvil cell, and data were obtained over the entire frequency range of FOX-7 Raman activity. Several new features were observed with increase of pressure: (i) new vibrational peaks and discontinuity in the shifts of the peaks at 2 and 4.5 GPa, (ii) apparent coupling or mixing of several modes, and (iii) changes in the NH2 stretching spectral shape and modes shift. The spectral changes at 2 GPa, in contrast to previous reports, involved only a few peaks and likely resulted from a small molecular transformation. In contrast, changes at 4.5 GPa involved most of the modes, and the pressure for the onset and completion of the changes depended on the pressure medium. A large pressure hysteresis regarding the changes at 4.5 GPa implies a reconstructive transformation. We suggest that this transformation reflects a change in the balance between interlayer (van der Waals) and in-layer (H-bonding) interactions. Despite these transformations, further compression to 40 GPa and subsequent release of pressure did not cause any irreversible changes. This finding implies that FOX-7 has remarkable chemical stability under high pressures. The observed coupling between the various modes with increasing pressure was analyzed within the Fermi resonance model. The potential implication of the coupling of modes for shock insensitivity of FOX-7 is briefly discussed.
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