The dynamics of nanomaterials and light metal deuterides studied by means of coherent inelastic neutron scattering measurements and model simulations

中子非弹性散射是研究固体中原子动力学的一种非常有效的技术。对于像氢这样的非相干散射体，测量产生状态的振动密度。对于相干散射材料，该实验获得了单元胞中原子对相对运动（相位和频率）的信息。对于单晶样品，例如用三轴光谱仪测量，测量得到声子色散曲线，该曲线可以直接与晶体中原子之间的力相关。然而，对于相干散射的多晶材料，声子频率在不同方向上的叠加产生了一个复杂的二维等高线图Q和omega（与中子碰撞中的动量转移和能量转移有关），迄今为止，只有少数实验试图揭示这个复杂的画面。然而，随着强大的计算机的发展，原子结构和相互作用的从头开始模拟以及灵活的软件工具的发展，可以为任何材料提供完整的晶格动力学模型，从另一个角度来解决问题变得实用，通过模拟多晶（poly-CINS）的相干非弹性散射并观察其与实验数据的匹配程度。我们已经开发了这种方法，以便分析石墨的聚cins，因为这种材料不是真正的单晶形式，并且已经证明现有的模型不能充分描述其晶格动力学。将这种方法应用于更复杂的碳的多晶态是特别有趣的。例如，我们的模型计算表明，单壁碳纳米管的聚cins数据应该产生沿管轴的杨氏模量值。事实上，这种方法将特别有趣地应用于通常具有纳米级结构的纳米材料。我们已经获得了关于天然石墨、碳纳米角和碳纤维的有趣数据，它们在非弹性散射中都显示出意想不到的结构，这需要大量的建模工作来解释。我们还将应用该技术来解释轻复合氢化物的散射，这些氢化物正在被研究作为汽车上可能使用的储氢材料。这些材料不是单晶，而且由于现有技术无法轻易解开氢键的性质，其他研究晶格动力学的方法也很复杂。利用氘化多晶体，我们应该能够解释非弹性散射，以验证从头计算，从而验证有限温度下的形成焓。一个主要目标是使该方法可用于国际上的中子散射。我们已经建立了一个广泛的合作者小组，他们将提供用于测量的样本，并将在数据解释方面进行合作。该软件是与加州理工学院富尔茨教授的团队合作开发的，该团队最近也开始朝着同样的目标努力。我们计划在第一年举办一个讲习班，为这些合作制定议定书，并在最后一年举办另一个讲习班，向国际中子散射中心介绍这种方法。

Neutron inelastic scattering is a very powerful technique for studying the dynamics of atoms in solids. For incoherent scatterers such as hydrogen, the measurement yields a vibrational density of states. For coherent scattering materials, the experiment yields information about the relative motion (both in phase and frequency) of pairs of atoms in the unit cell. With a single crystal sample, measured for instance on a Triple Axis Spectrometer, the measurement yields phonon dispersion curves which can be directly related to the forces between atoms in the crystal. However, for coherently scattering polycrystalline materials, the superposition of phonon frequencies in different directions yields a complex 2-D contour plot in Q and omega ( related to momentum transfer and energy transfer in the collision with the neutron) and to date, there have only been a handful of experiments attempting to unravel this complex picture. However, with the development of powerful computers, of ab initio simulation of atomic structures and interactions and flexible software tools that can provide complete models of the lattice dynamics for any material, it becomes practical to approach the problem from the other point of view, by simulating the coherent inelastic scattering from a polycrystal (poly-CINS) and seeing how well it matches the experimental data. We have developed this method so as to analyse the poly-CINS from graphite as this material is not available in true single crystalline form, and have demonstrated that the existing models are not adequate descriptions of its lattice dynamics. It is particularly interesting to apply this method to the more complex polymorphs of carbon. For instance, our model calculations demonstrate that poly-CINS data for single walled carbon nanotubes should yield a value for Young's Modulus along the tube axis. Indeed, the method will be particularly interesting to apply to nanomaterials in general where the material has a structure on the nanoscale. We have already obtained interesting data on natural graphite, carbon nanohorns and carbon fibres which all show unexpected structure in the inelastic scattering that will require considerable modeling effort to interpret. We will also apply the technique to the interpretation of scattering from light complex hydrides which are being investigated as possible hydrogen storage materials for use on cars. These materials are not available as single crystals and other methods of investigating the lattice dynamics are complicated because of the nature of the H-H bonding which existing techniques cannot easily unravel. Using deuterated polycrystals, we should be able to interpret the inelastic scattering to validate the ab initio calculations and hence the enthalpy of formation at finite temperaturesA major objective is to make the method available to neutron scatterers internationally. We have already set up an extensive group of collaborators who will provide samples for measurement and will collaborate in the data interpretation. The software is being developed in collaboration with Prof. Fultz's group at Caltech, which has recently started working towards the same objective. We plan a workshop in the first year to establish the protocol for these collaborations and a further workshop in the final year to introduce the method to the international neutron scattering centres.