Nuclear magnetic resonance study of hydrogen mobility in LaY2Ni9Hx and CeY2Ni9Hx

In order to investigate the mobility of hydrogen in the ternary compounds LaY2Ni9 and CeY2Ni9, we have measured the proton nuclear magnetic resonance (NMR) spectra and the proton spin–lattice relaxation rates R1 for LaY2Ni9Hx (x=0.78 and 10.3) and CeY2Ni9Hx (x=0.79 and 7.2). The measurements have been performed at the resonance frequencies of 14, 23.8 and 90 MHz over the temperature range of 11–404 K. For LaY2Ni9Hx, the behavior of R1 is consistent with the presence of a considerable contribution due to motionally modulated dipole–dipole interaction between nuclear spins. This contribution can be satisfactorily described in terms of the Bloembergen–Purcell–Pound model with a Gaussian distribution of activation energies for hydrogen diffusion; the value of the average activation energy resulting from such a description for LaY2Ni9H10.3 is approximately 0.36 eV. For CeY2Ni9Hx compounds, the measured proton spin–lattice relaxation rates are dominated by the contribution due to the interaction between proton spins and paramagnetic centers over the entire temperature range studied. Only qualitative information on H mobility in these compounds has been obtained from the narrowing of NMR spectra. Possible paths of H diffusion in LaY2Ni9Hx and CeY2Ni9Hx are discussed in terms of the distances between the interstitial sites occupied by hydrogen.