D. Keith Ross

Pressure-dependent deuterium reaction pathways in the Li-N-D system

Neutron diffraction data from in situ deuteration and dedeuteration of Li(3)N are presented under different pressure regimes, whereby reaction pathways differing from the widely reported stoichiometric pathway of Li(3)N + 2D(2)<--> Li(2)ND + LiD + D(2)<--> LiND(2) + 2LiD are observed. At sufficiently high pressures, where the deuterium chemical potential is comparable with the heat of amide formation, the reaction appears to be driven straight to the amide plus deuteride phase mixture. At lower pressures, a cubic phase exhibiting a concentration-dependent variation in lattice parameter is observed. In dedeuteration, two sets of reflections from cubic structures with distinct lattice parameters are observed, both of which exhibit a continual decrease in cell volume. The reaction pathways are discussed in terms of the compositional variation.

In situ powder neutron diffraction study of non-stoichiometric phase formation during the hydrogenation of Li3N.

The hydrogenation of Li3N at low chemical potential has been studied in situ by time-of-flight powder neutron diffraction and the formation of a non-stoichiometric Li4-2xNH phase and Li4NH observed. The results are interpreted in terms of a model for the reaction pathway involving the production of Li4NH and Li2NH, which subsequently react together to form Li4-2xNH. Possible mechanisms for the production of Li4NH from the hydrogenation of Li3N are discussed.

Inelastic and Quasi-Elastic Neutron Scattering

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.

The interpretation of polycrystalline coherent inelastic neutron scattering from aluminium

Presented here is a method by which Q-dependent dispersive dynamics may be measured (and used to generate a lattice dynamical model) for polycrystalline samples. This method is analogous to the measurement of dispersion curves for single-crystal samples.