Cross relaxation between proton and quadrupolar nuclear spins in metal-hydrogen systems.

Abstract

Enhanced proton-NMR spin-lattice-relaxation rates ${\mathit{R}}_{1}$ measured over a range of frequencies and temperatures in the Nb-H, Nb-V-H, and Ta-H metal-hydrogen systems are attributed to dipolar cross relaxation between the Zeeman energy levels of the proton spin and the combined Zeeman-quadrupole levels of the metal nuclear spin. This cross-relaxation mechanism can dominate the total ${\mathit{R}}_{1}$ at low temperatures even in static (nonrotating) polycrystalline samples, resulting in relaxation rates up to 400 times greater than those expected from electronic relaxation alone. At low temperatures, the relaxation rates decreased roughly linearly with temperature, in some cases extrapolating to zero at 0 K, and in other cases having a finite intercept at 0 K. We calculate the cross-relaxation rate for the cases of Nb-H and Ta-H and compare the results with experimental data. A crucial feature of our interpretation is that, even in the ordered hydrides, there is substantial structural disorder. Although experiments were limited to proton relaxation in metal-hydrogen systems, our conclusions about the effects of cross relaxation on the total spin-lattice-relaxation rate should apply equally well to other heteronuclear spin systems in solids having some degree of structural disorder.