The Effect of Magnetic Impurities on the Apparent Diffusion Coefficient of Hydrogen in Metal Hydrides Deduced from NMR
Observations of the well-known minimum in proton spin-lattice relaxation time, T1, as a function of temperature, T, caused by diffusion-induced fluctuations of nuclear dipole-dipole interactions, have often been used to determine hydrogen diffusion coefficients, D, in metal-hydrogen systems. In several cases the minimum has appeared asymmetric with a shallower low-temperature slope interpreted as a change to a diffusion mechanism with a lower activation energy. In a few cases a subsidiary low-temperature minimum has been observed or inferred and interpreted in terms of two coexisting mechanisms. We believe that these complications are likely to be due to interaction with paramagnetic impurities, analogous to the case of doped fast-ion conductors. We have measured T1(T) for H in YH1.98 containing controlled amounts of gadolinium and observed growth of the above features with increasing impurity content, together with a low-temperature region (T1 independent of T) controlled by spin-diffusion, and also the eginning of a third, high-temperature minimum not previously found in metal-hydrogen systems. It is concluded that some D(T) values deduced from existing NMR data must be treated with caution until measurements on specimens with controlled impurity content are complete.
KeywordsApparent Diffusion Coefficient Hyperfine Field Metal Hydride Magnetic Impurity Paramagnetic Impurity
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