Dreger Zbigniew A, Gupta Yogendra M
Institute for Shock Physics and Department of Physics, Washington State University, Pullman, Washington 99164-2816, USA.
J Phys Chem B. 2007 Apr 19;111(15):3893-903. doi: 10.1021/jp0681092. Epub 2007 Mar 23.
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.
为深入了解含能晶体黑索今（RDX）的高压多晶型，获得了其在静水压力（高达15吉帕）和非静水压力（高达22吉帕）压缩下的拉曼光谱结果。在4.0±0.3吉帕处发现光谱有几个明显变化，证实了先前在多晶样品中观察到的α-γ相变。对内部和外部（晶格）模式压力诱导变化的详细分析揭示了4吉帕以上的几个特征：（i）A'和A''内部模式均分裂，（ii）NO2模式的拉曼位移压力依赖性显著增加，（iii）外部模式数量无明显变化。有人提出，α-γ相变导致RDX分子之间重新排列，进而显著改变N-O键所经历的分子间相互作用。对称相关性分析表明，γ-多晶型可能具有三种正交结构之一：D2h、C2v或D2。根据现有的X射线数据，D2h因子群比其他结构更受青睐，有人提出γ相RDX具有与点群D2h同构的空间群，八个分子占据C1对称位点，类似于α相。据信，因子群分裂可以解释γ相中观察到的模式数量增加。在高达22吉帕的非静水压缩晶体中对拉曼模式进行空间映射，发现模式位置存在很大差异，表明晶体中存在压力梯度。在非静水压缩下未观察到拉曼光谱有明显的不可逆变化。
