| Abstract
| - To gain insight into the high-pressure polymorphism of RDX, an energetic crystal, Raman spectroscopy resultswere obtained for hydrostatic (up to 15 GPa) and non-hydrostatic (up to 22 GPa) compressions. Severaldistinct changes in the spectra were found at 4.0 ± 0.3 GPa, confirming the α−γ phase transition previouslyobserved in polycrystalline samples. Detailed analyses of pressure-induced changes in the internal and external(lattice) modes revealed several features above 4 GPa: (i) splitting of both the A‘ and A‘ ‘ internal modes, (ii)a significant increase in the pressure dependence of the Raman shift for NO2 modes, and (iii) no apparentchange in the number of external modes. It is proposed that the α−γ phase transition leads to a rearrangementbetween the RDX molecules, which in turn significantly changes the intermolecular interaction experiencedby the N−O bonds. Symmetry correlation analyses indicate that the γ-polymorph may assume one of thethree orthorhombic structures: D2h, C2v, or D2. On the basis of the available X-ray data, the D2h factor groupis favored over the other structures, and it is proposed that γ-phase RDX has a space group isomorphous witha point group D2h with eight molecules occupying the C1 symmetry sites, similar to the α-phase. It is believedthat the factor group splitting can account for the observed increase in the number of modes in the γ-phase.Spatial mapping of Raman modes in a non-hydrostatically compressed crystal up to 22 GPa revealed a largedifference in mode position indicating a pressure gradient across the crystal. No apparent irreversible changesin the Raman spectra were observed under non-hydrostatic compression.
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