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

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.