In situ studies of the transition from NaAlH4 to Na3AlH6 are performed by proton NMR for samples doped with TiCl3 and with Ti-13-nanoclusters. The local hydrogen dynamics in the different compounds is studied by the nuclear spin-lattice relaxation. For the Ti-doped NaAlH4 samples a double-exponential recovery of the nuclear magnetization is observed, indicating two fractions of hydrogen with different mobilities. In Na3AlH6 a transition from hindered rotation of the AlH6 groups to full isotropic reorientation of these groups is observed in the temperature range 200-260 K. (c) 2005 Elsevier B.V. All rights reserved.

Hydrogen-loaded nanocrystalline graphite samples have been prepared by mechanical milling under a hydrogen atmosphere. Milling vials and balls made of agate and ZrO2 have been used to prepare samples with hydrogen contents between 1 and 2wt%. The proton nuclear-magnetic-resonance (H1-NMR) spectra of these samples are well represented by the sum of a broad Gaussian and a more narrow Lorentzian line corresponding to hydrogen in C–H covalent bonds as well as to hydrogen in methyl groups. The temperature dependence of the Lorentzian line can be ascribed to a hindered rotation of the methyl groups. The corresponding activation enthalpy of about 0.12eV has been deduced from the spin-lattice relaxation rates between 250 and 450K. Below about 200K the relaxation rates are temperature independent but they depend strongly on the NMR frequency and on the parameters of the sample preparation. The relaxation due to paramagnetic impurities as well as the cross relaxation of the proton spins with spins of quadrupolar impurity nuclei are proposed to contribute significantly to the measured spin-lattice relaxation rates.

The diffusion mechanisms of hydrogen in metallic and nanostructured materials have been studied systematically by different nuclear magnetic resonance techniques. The present paper reviews three examples of our recent work: (i) The hydrogen-stabilized Laves-phase compound C15-HfTi2H4, with rather complex mechanisms of hydrogen diffusion. Long-range diffusion and localized motion coexist on different time scales in this compound. (ii) Nanostructured vanadium-hydrides n-VHx, in which the dynamical properties of hydrogen are fundamentally changed compared to that in a crystalline compound. The diffusion parameters of hydrogen in the grain boundary regions could be determined independently of the hydrogen motion inside the crystalline grains. (iii) Hydrogen in nanostructured hydrogen-graphite-systems n-CHx, where the NMR spectra reveal two types of hydrogen coordinations. The relaxation data indicate high hydrogen mobilities at ambient temperatures.

Nanostructured hydrogen–graphite systems, CnanoHx (x=0.24, 0.31, 0.96), have been characterized by first nuclear magnetic resonance (NMR) measurements. The NMR spectrum of CnanoH0.96 is well represented by the sum of a Lorentzian and a Gaussian line, indicating two types of hydrogen coordinations. These two components may be ascribed to hydrogen in graphite interlayers and hydrogen chemisorbed at dangling bonds. Information on the hydrogen hopping frequencies is provided by the spin–lattice relaxation rate Γ1. The temperature dependence of Γ1 yields high hydrogen diffusivities and low activation energies of Ea≈0.1 eV. A change in the Γ1 data of CnanoHx with x=0.24 and 0.31 occurred after the samples had been heated to about 400–430 K. This suggests that in this temperature range hydrogen atoms start to occupy sites with different site energies, resulting in a distribution of the activation energies for hydrogen motion.