黑索金的非弹性中子散射及第一性原理计算

Inelastic neutron scattering and ab initio studies of cyclotrimethylenetrinitramine

黑索金(环三亚甲基三硝胺,RDX,C_3H_6O_6N_6)是一种非常重要的次级炸药,因其高能量密度及对外界刺激的低感度而具有广泛的军事和工业应用.为了能在生产、运输、存储以及使用中对其行为进行有效控制,人们对它的化学性质、力学性质,尤其是起爆进行了大量的研究.炸药的起爆是一个非常复杂的过程,其中最主要的问题之一就是能量是如何从连续介质尺度的刺激转移到原子尺度引起吸热分解的.根据冲击波致爆的非平衡态Zel’dovich-von Neumann-Doering模型,声子作为最初的热载体在整个过程中起着非常重要的作用.实验上,非弹性中子散射技术是研究晶体中原子和分子运动动力学的有力手段,尤其是对于包含了大部分声子晶格模式的低频区域来说极具优势.利用非弹性中子散射技术测得了RDX在10—104 cm^(-1)范围内的振动谱,结合固态量子化学计算,对所测的12个振动模式进行指认.研究结果有助于人们对起爆详细机理的认识.

As an important secondary explosive, cyclotrimethylenetrinitramine （RDX, C3H6O6N6） is extensively used in mili- tary and industrial applications due to its high energy density and low sensitivity to external stimulations. Considerable attention has been devoted to the study of the detonation initiation, with particular interest in the mechanism by which energy is transferred from a shock wave to the internal molecular vibrations so as to begin endothermic decomposition. During the whole process, phonons as the primary carriers of heat may play an important role. Experimentally, inelastic neutron scattering （INS） technique provides a means of studying the dynamics of motions of atoms and molecules in the crystal, especially in the low frequency region which contains most phonon lattice modes. In this work, neutron diffraction pattern of polycrystalline RDX under ambient condition has been measured and compared with the calculated results, showing reasonable agreement and thus confirming the structure of RDX. Subsequently, the vibrational INS spectrum of RDX has been measured at T = 10 K over the region of 10-104 cm^-1 by using cold neutron triple-axis spectrometer. On the basis of the solid-state density functional calculations with the generalized gradient approximation （BLYP and BP functionals）, it is possible to perform normal-mode analysis, which agrees with previous assignments. A total of 9 phonon lattice modes and 3 internal vibrations have been identified. Eight possible ＂doorway modes＂ may be predicted in the energy range between 100 and 148 cm-1, which arise from the combinations of phonon lattice modes 38.3, 40.3, 50.2, 61.5 cm-1 and fundamental vibrations 86.6, 88.6, 101.4 cm-1. The doorway modes are the proposed bridge by which the energy of initial shoekwave can pass from the external degrees of freedom into those of the molecule. It is shown that all of these eight modes have fundamental vibrational components that consist of nitro-group deformation vibrations. This point is of particular importance and supports the theory that the initial bond broken in detonation is the N--N bond. This work may shed light on the mechanism of detonation initiation from a microscopic viewpoint.