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方法。对于非相干散射体，如氢，测量产生振动态密度。对于相干散射材料，该实验产生了关于单位晶胞中原子对的相对运动（相位和频率）的信息。对于单晶样品，例如在三轴光谱仪上测量，测量产生声子色散曲线，其可以与晶体中原子之间的力直接相关。然而，对于相干散射的多晶材料，不同方向上的声子频率的叠加产生了Q和Ω（与中子碰撞中的动量转移和能量转移有关）的复杂的2-D等高线图，并且迄今为止，只有少数实验试图解开这个复杂的图像。然而，随着功能强大的计算机的发展，原子结构和相互作用的从头计算模拟以及灵活的软件工具，可以为任何材料提供完整的晶格动力学模型，从另一个角度来处理这个问题变得实用，通过模拟多晶体（poly-CINS）的相干非弹性散射，看看它与实验数据的匹配程度。我们已经开发了这种方法，以分析聚CINS从石墨，因为这种材料是不是在真正的单晶形式，并已证明，现有的模型是不充分的描述其晶格动力学。将这种方法应用于碳的更复杂的多晶型物是特别有趣的。例如，我们的模型计算表明，单壁碳纳米管的poly-CINS数据应产生沿管轴沿着的杨氏模量值。事实上，该方法将特别有趣地应用于一般的纳米材料，其中材料具有纳米级的结构。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，这些数据都在非弹性散射中显示出意想不到的结构，这将需要相当大的建模工作来解释。我们还将应用该技术的解释散射光复杂的正被调查作为可能的储氢材料用于汽车。这些材料不能作为单晶获得，并且研究晶格动力学的其他方法由于现有技术不能容易地解开的H-H键合的性质而复杂。使用氘化多晶体，我们应该能够解释的非弹性散射，以验证从头计算，从而在有限的temperaturesA的主要目标是使该方法可用于中子散射国际上的生成焓。我们已经建立了一个广泛的合作者小组，他们将提供测量样本，并将在数据解释方面进行合作。该软件正在与加州理工学院的Fultz教授小组合作开发，该小组最近开始朝着同一目标努力。我们计划在第一年举办一个研讨会，为这些合作建立协议，并在最后一年举办一个研讨会，向国际中子散射中心介绍该方法。

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.