NEUTRON QUASI-ELASTIC SCATTERING MEASUREMENTS OF HYDROGEN DIFFUSION IN THE NIZR2 INTERMETALLIC PHASE

Neutron quasi-elastic scattering measurements have been made on a powd ered sample of the crystalline intermetallic phase NiZr2 containing in terstitial hydrogen at a hydrogen-to-metal ratio of 1.6. The experimen t was conducted over a range of momentum transfers from 0.4 to about 2 Angstrom(-1) and at temperatures from 400 to 460 K. It was found that about 80% of the spectral area could be accounted for by a lorentzian function with an energy halfwidth of 1-10 mu eV depending on the mome ntum transfer and temperature. The addition of a further lorentzian fu nction with a halfwidth about an order of magnitude less was required to give the true shape of the spectrum. The temperature dependence of the spectral widths are consistent with activation energies of 0.32 an d 0.395 eV. The relationship between the quasi-elastic spectrum and th e diffusion of the hydrogen is discussed in the light of hopping rates and jump lengths derived from nuclear magnetic relaxation measurement s and the known structural properties of the alloy. The alloy NiZr2 is of particular interest because amorphous Ni-Zr alloys can also be for med near this composition. In spite of similarities in the distributio n of metal atoms around the sites occupied by hydrogen the diffusion p roperties of the amorphous alloys, as derived from earlier quasi-elast ic measurements, are very different. Comparisons are drawn between the spectral shapes in the amorphous and crystalline state.