The theory of nuclear spin relaxation was developed for a random-walk model of H diffusing between sites that formed a non-Bravais lattice structure; including multiple rates of jumps between sites in a unit cell. The results were applied to H diffusing via first- and second-nearest neighbour jumps between interstitial d-sites in the A15 compound, Nb3AlHx. The random-walk model was exact for H-metal dipolar relaxation in the low H-concentration limit and provided a good approximation at arbitrary concentrations when average site-blocking of jumps was included. This model could exhibit the high-temperature 1/√ω frequency dependence of the relaxation rates, for 1-dimensional diffusion, which were possible in this structure for nearest-neighbour jumps along the crystal axes. The low- and high-H concentration proton-relaxation data for Nb3AlHx, as a function of temperature and resonance frequency, were fitted well at high temperatures by using a simple set of H jump parameters. The theory provided a more rigorous approach and permitted a more complete analysis than did the simple Bloembergen–Purcell–Pound model used in previous work.

Nuclear Spin Relaxation and Diffusion of Hydrogen in the A15 Compound Nb3AlHx. C.A.Sholl: Journal of Physics - Condensed Matter, 2007, 19[40], 406228 (8pp)