Inelastic and Quasi-Elastic Neutron Scattering

Abstract

This chapter describes the basic principles of inelastic and quasi-elastic neutron scattering as applied to hydrogen storage systems—in particular to provide an understanding of the vibrations and diffusion of hydrogen and deuterium in the host lattice. The techniques are then illustrated using typical hydrogen storage materials. Incoherent inelastic scattering can be applied to isolated hydrogens—where the protons can be modelled as in an isolated potential well formed by the surrounding atoms. At higher concentrations, the effect of H–H interactions and the role of hydrogen vibrational density of states are described. Ab initio theory becomes important in this case. The advantages of modelling the dynamics of a deuteride by simulation of the polycrystalline coherent inelastic neutron scattering in comparison with ab initio modelling are then described. The final area of application of inelastic scattering is to the case of adsorbed H2 molecules where particular spin transitions are observed. Here the results provide important information on the geometry of the potential energy surface around the adsorbing site. Quasi-elastic neutron scattering is then described. In particular the Chudley–Elliott model is derived for a Bravais lattice and it is indicated how this approach can be extended to more general cases where there are multiple sublattices which may have differing energies of adsorption. Here the important case of intermetallic Laves phases is described.