Kearley G J, Johnson M R, Tomkinson J
Department of Radiation, Radionuclides and Reactors, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands.
J Chem Phys. 2006 Jan 28;124(4):044514. doi: 10.1063/1.2145926.
The lattice dynamics and molecular vibrations of benzene and deuterated benzene crystals are calculated from force constants derived from density-functional theory (DFT) calculations and compared with measured inelastic neutron-scattering spectra. A very small change (0.5%) in lattice parameter is required to obtain real lattice-mode frequencies across the Brillouin zone. There is a strong coupling between wagging and breathing modes away from the zone center. This coupling and sensitivity to cell size arises from two basic interactions. Firstly, comparatively strong interactions that hold the benzene molecules together in layers. These include an intermolecular interaction in which H atoms of one molecule link to the center of the aromatic ring of a neighboring molecule. The layers are held to each other by weaker interactions, which also have components that hold molecules together within a layer. Small changes in the lattice parameters change this second type of interaction and account for the changes to the lattice dynamics. The calculations also reveal a small auxetic effect in that elongation of the crystal along the b axis leads to an increase in internal pressure in the ac plane, that is, elongation in the b direction induces expansion in the a and c directions.
从密度泛函理论（DFT）计算得出的力常数出发，计算了苯和氘代苯晶体的晶格动力学和分子振动，并与测量的非弹性中子散射光谱进行了比较。为了在整个布里渊区获得真实的晶格模式频率，晶格参数需要有非常小的变化（0.5%）。远离区中心时，摇摆模式和呼吸模式之间存在强耦合。这种耦合以及对晶胞尺寸的敏感性源于两种基本相互作用。首先，是将苯分子在层内结合在一起的相对较强的相互作用。这些相互作用包括一种分子间相互作用，其中一个分子的H原子与相邻分子芳香环的中心相连。各层通过较弱的相互作用相互维系，这些相互作用也有将分子在层内结合在一起的成分。晶格参数的微小变化会改变这种第二类相互作用，并解释了晶格动力学的变化。计算还揭示了一种小的负泊松比效应，即晶体沿b轴伸长会导致ac平面内的内压增加，也就是说，在b方向上的伸长会引起a和c方向上的膨胀。
