单轴压缩下固态硝基苯的第一性原理研究

First-principle simulation of solid nitrobenzene under uniaxial compression

采用第一性原理密度泛函理论结合经典色散修正方法,对固态硝基苯在单轴压缩下的基本结构关系进行了计算.静水压缩和单轴压缩都压缩到初始平衡体积的70%.将静水压下优化后的晶胞体积、晶格参数以及平衡条件下的晶格能与实验值进行了比较,均符合较好.同时,为了充分地表征固态硝基苯的各向异性,将硝基苯沿着三个晶格矢量的方向进行单轴压缩,把每个方向的应力张量、能带带隙、每个原子能量的改变分别作为体积压缩比的函数进行了比较和分析.其中,最显著的各向异性效应是在体积压缩比为0.76时,沿X轴压缩导致硝基苯能带带隙闭合,体系呈金属化;而静水压缩或沿Y轴和Z轴压缩时体系始终呈半导体状态,带隙均大于1.59 eV.为了充分理解这一各向异性特性,我们计算了硝基苯晶体的局域态密度和电荷密度分布,并对金属化现象做出了合理的分析和解释.在不同的压力加载条件下,通过对不同物理量的计算,发现X轴方向是硝基苯晶体内部最敏感的方向.这些各向异性效应的研究将有助于人们在原子尺度上深入理解固态硝基苯的物理化学性质.

Energetic materials（EMs） including explosives, propellants and pyrotechnics have been widely used for the military and many other purposes. Solid nitrobenzene（an organic molecular crystal） could be considered as a prototype of energetic material. Up to now, numerous studies have been devoted to crystal structures, spectrum properties and decomposition mechanisms for solid nitrobenzene experimentally and theoretically. However there has been a lack of the comprehensive understanding of the anisotropic characteristics under different loading conditions. Thus we investigate the hydrostatic and uniaxial compressions along three different lattice directions to determine this anisotropic effect.In this work, the density functional theory calculations are performed based on Cambridge Sequential Total Energy Package（CASTEP） code using normconserving pseudo potentials and a kinetic energy cutoff of 700 e V. The generalized gradient approximation with the Perdew-Burke-Ernzerhof parameterization is used. Monkhorst-Pack k-point meshes with a density of 0.05 ？^（-1） are used for Brillouin-zone integration. The empirical dispersion correction by Grimme is taken to account for week intermolecular interactions. The hydrostatic compressions are applied from 0 GPa to 20 GPa. Cell volume, lattice shape and coordinates of the atoms could be fully relaxed. while uniaxial compression is applied up to 70% of the equilibrium cell volume in steps of 2% along their lattice directions respectively. At each compression step, only atomic coordinates are allowed to relax, with the lattice fixed. The equilibrium lattice structures under hydrostatic compressions are obtained by full relaxation at 0 K temperature. In ambient condition, the calculated volume and parameter of the unit cell are underestimated compared with the experimental data, and corresponding errors are -2.98%, 0.01%,-4.39%, 5.71% respectively. In contrast, the calculated lattice energy is overestimated compared with the range of experimental results with 5.71% of the error. In high pressure condition, the volume and cell parameter of the unit cell as a function of compression ratio are plotted and compared with the experimental data. The theoretical and experimental values are close with the increase of the pressure, for instant, the error decreases from -4.39% at 0 GPa to -1.93% at 4 GPa. On the other hand, the uniaxial compression is applied along the directions of three lattice vectors. The changes of stress tensor, band gap, energy per atom as a function of compression ratio are also plotted and discussed, which can characterize the anisotropic effect of solid nitrobenzene. The most noticeable effect of anisotropy in solid nitrobenzene is the metallization at V/V_0= 0.76 compressed along the X axis, while the solid nitrobenzene under hydrostatic pressure or other uniaxial compressions up to V/V_0= 0.76 remains semiconductor with band gap larger than 1.591 eV. By analyzing the local density of states and charge density distribution of nitrobenzene crystal, we confirm that the metallization is caused by the overlap of the π electron from benzene ring. Through calculating different physical parameters, we find that X axis is the most sensitive direction of nitrobenzene crystal. The studies of anisotropic effects are expected to shed light on the physical and chemical properties of solid nitrobenzene on an atomistic scale and provide several insights for experiments.