On the NMR determination of proton diffusion mechanisms in transition metal hydrides

Abstract

The effect of proton diffusion in transition metal hydrides on (i) the anisotropies of the nuclear spin relaxation times T1, T2, and T1 rho in single crystals and (ii) on the width and shape of high-field T1 and T1 rho minima in powdered samples is investigated. To illustrate the basic ideas, the simplest case of unmagnetic FCC host metals (e.g., palladium), in which either tetrahedral or octahedral interstitial sites are assumed to be randomly occupied, is considered in detail. It is found that the anisotropies of T1, T2, and T1 rho and of the dipolar local field in the rotating frame (the latter being proportional to the second moment of the absorption line) are extremely sensitive functions of the particular sublattice predominantly occupied by protons. It is shown that a vacancy mechanism (encountered perhaps for large proton concentrations) leads to narrower T1 and T1 rho minima than a random-walk mechanism of diffusion on the two sublattices considered.