High Pressure Raman Spectroscopy of Single Crystals of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)

Abstract

To gain insight into the high-pressure polymorphism of RDX, an energetic crystal, Raman spectroscopy results were obtained for hydrostatic (up to 15 GPa) and non-hydrostatic (up to 22 GPa) compressions. Several distinct changes in the spectra were found at 4.0 +/- 0.3 GPa, confirming the alpha-gamma phase transition previously observed 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 apparent change in the number of external modes. It is proposed that the alpha-gamma phase transition leads to a rearrangement between the RDX molecules, which in turn significantly changes the intermolecular interaction experienced by the N-O bonds. Symmetry correlation analyses indicate that the gamma-polymorph may assume one of the three orthorhombic structures: D2h, C2v, or D2. On the basis of the available X-ray data, the D2h factor group is favored over the other structures, and it is proposed that gamma-phase RDX has a space group isomorphous with a point group D2h with eight molecules occupying the C1 symmetry sites, similar to the alpha-phase. It is believed that the factor group splitting can account for the observed increase in the number of modes in the gamma-phase. Spatial mapping of Raman modes in a non-hydrostatically compressed crystal up to 22 GPa revealed a large difference in mode position indicating a pressure gradient across the crystal. No apparent irreversible changes in the Raman spectra were observed under non-hydrostatic compression.

Density functional theory calculations of anisotropic constitutive relationships in alpha-cyclotrimethylenetrinitramine

“…2 The hydrostatic equation of state ͑EOS͒ of RDX at room temperature was obtained by Olinger et al 3 and Yoo et al 4 Both experiments 3,4 found a phase transition from ␣-RDX to ␥-RDX near 4 GPa. Subsequently, Raman spectroscopy has been employed for a detailed investigation of RDX at pressures up to about 4 GPa by Ciezak et al 6 and Dreger and Gupta, 7 who found features in the spectra that are associated with the phase transition at this pressure. Very recently, the structure of the ␥ polymorph has been reported by Davidson et al 8 using a combination of x-ray and neutron diffraction.…”

Recent Advances and Remaining Challenges for the Spectroscopic Detection of Explosive Threats

“…48,49 In the solid state, RDX adopts a crystalline a phase with an AAE chair conformation in which two of the nitro groups point along the axis of the ring, and the third points in a nearly equatorial direction. 49,50 The Raman spectra and crystal structure have been studied in detail and vibrational mode assignments suggested. [50][51][52] The HMX is similar to RDX except that it contains an eight-membered ring instead of a six-membered ring.…”

“…49,50 The Raman spectra and crystal structure have been studied in detail and vibrational mode assignments suggested. [50][51][52] The HMX is similar to RDX except that it contains an eight-membered ring instead of a six-membered ring. The most stable phase is denoted as b-HMX and is the one that is present in bulk explosives.…”

Energetic materials under high pressures and temperatures: stability, polymorphism and decomposition of RDX

“…The - phase transition is well established under static compression and recognized by characteristic changes in the Raman spectra [9]. However, it was unclear if this phase transition can occur under shock compression conditions.…”

“…Given that the behaviour of RDX at high pressures and temperatures is of primary interest for understanding the shock initiation, the proper characterization of the HP-HT phase is essential. Therefore, in this work we present recent studies on RDX at high pressures and temperature to address the previous inconsistencies and to complement understanding of processes under shock compression [1,2,[9][10][11][12]. In particular, we focus on: (i) determining details of the vibrational structure of the HP-HT phase, (ii) assessing the claimed association of this phase with -RDX, (iii) providing the refined P-T diagram of RDX, (iv) examining the stability and decomposition of the HP-HT phase, and (v) relating the static compression results to those under shock compression.…”