Nuclear spin relaxation and diffusion of hydrogen in the A15 compoundNb3AlHx

Abstract

The theory of nuclear spin relaxation is developed for a random walk model of H diffusingbetween sites that form a non-Bravais lattice structure, including multiple rates of jumpsbetween sites in a unit cell. The results are applied to H diffusing by first andsecond nearest neighbour jumps between interstitial d sites in the A15 compoundNb3AlHx. The random walk model is exact for H–metal dipolar relaxation in the low Hconcentration limit and provides a good approximation at arbitrary concentrations whenaverage site-blocking of jumps is included. This model can show the high temperature frequency dependence of the relaxation rates for one-dimensional diffusionpossible in this structure for nearest neighbour jumps along the crystalaxes. The low and high H concentration proton-relaxation data forNb3AlHx, as a function of temperature and resonance frequency, are fitted well at high temperaturesusing a simple set of H jump parameters. The theory provides a more rigorous approachand enables a more complete analysis than the simple Bloembergen–Purcell–Pound modelused in previous work.