Nuclear spin relaxation and diffusion of hydrogen in the A15 compound Nb₃AlH_x

Abstract

The theory of nuclear spin relaxation is developed for a random walk model of H diffusing between sites that form a non-Bravais lattice structure, including multiple rates of jumps between sites in a unit cell. The results are applied to H diffusing by first and second nearest neighbour jumps between interstitial d sites in the A15 compound Nb(3)AlH(x). The random walk model is exact for H-metal dipolar relaxation in the low H concentration limit and provides a good approximation at arbitrary concentrations when average site-blocking of jumps is included. This model can show the high temperature [Formula: see text] frequency dependence of the relaxation rates for one-dimensional diffusion possible in this structure for nearest neighbour jumps along the crystal axes. The low and high H concentration proton-relaxation data for Nb(3)AlH(x), as a function of temperature and resonance frequency, are fitted well at high temperatures using a simple set of H jump parameters. The theory provides a more rigorous approach and enables a more complete analysis than the simple Bloembergen-Purcell-Pound model used in previous work.

Random walk theory of1H spin relaxation in TaV2Hx

“…Random walk theory of nuclear spin relaxation has previously been applied to hydrogen diffusion on Bravais lattices using a reciprocal space formalism [13]. This method has recently been extended to diffusion on interstitial sites in the cubic A15-type structure Nb 3 AlH x which form a non-Bravais lattice structure with multiple jump rates [14]. The reciprocal space method is, however, quite inefficient for the C15 structure because of the crystal structure and the large number (12) of g sites per unit cell.…”