H1 NMR analysis of nuclear relaxation mechanisms in Pd-H and Pd-Ag-H alloys

Crystalline Pd1−xAgx-Hy (x=0-0.35) alloys were studied as model systems representing a chemically disordered system for hydrogen-storage materials. Extensive H1 relaxation rate (R1, R1ρ and R2) measurements were carried out in a wide temperature range of 2.4-400 K. Palladium-silver alloys of silver concentration as high as 35 at.% were studied by NMR spectroscopy for the first time. The relaxation mechanisms magnetic dipolar relaxation (hydrogen diffusion), Korringa relaxation, relaxation by paramagnetic impurity ions and cross-relaxation to quadrupolar metal nuclei were identified and characterized. The presence of cross-relaxation between H1 and quadrupolar Pd105 nuclear spins was assumed and demonstrated for the first time in Pd-H and Pd1−xAgx-H systems. More than one jump processes of diffusing hydrogen were found to be present in the studied palladium-silver alloys. Only lower estimated limit values of Korringa constants could be obtained since no frequency independent R1 versus T region was found. These values are significantly higher than the ones reported by others.