Nuclear magnetic resonance study of hydrogen diffusion in A15-type Nb3AlHx

Nuclear magnetic resonance measurements of the proton spin-lattice relaxati on rate T-1(-1) for A15-type Nb3AlHx (x = 0.13, 1.77 and 2.75) have been pe rformed over the temperature range 20-420 K and the resonance frequency ran ge 13-90 MHz. The experimental results are analysed to evaluate the electro nic (Korringa) contributions to T-1(-1) and the parameters of hydrogen diff usion. It is found that the electronic contribution to the relaxation rate decreases with increasing H content. The effective activation energies for H diffusion derived from the high-temperature relaxation rate data are 0.21 eV for x = 0.13 and 0.30 eV for x = 1.77 and 2.75. The behaviour of T-1(-1 ) at low temperatures (T < 270 K for x = 1.77 and 2.75) suggests a coexiste nce of at least two frequency scales of H hopping. It is shown that the str ucture of the sublattice of interstitial d sites (partially occupied by hyd rogen) is consistent with the coexistence of two characteristic jump rates for H atoms.