Hydrogen Diffusion in f.c.c. TiH _<i>x</i> and YH _<i>x</i> : Two Distinct Examples for Diffusion in a Concentrated Lattice Gas

Abstract

The self-diffusion coefficient of hydrogen in the dihydrides TiHx and YHx was investigated by neutron spectroscopy (1.59 < x < 2.09, 350 °C < T < 950 °C). The two hydrides differ in that hydrogen in TiHx occupies solely tetrahedral interstitial sites, whereas a small percentage (< 10%) of the hydrogen in YHx is also located on octahedral sites. The measurements demonstrate that these (small) differences in the interstitial occupancies cause a distinctly different concentration dependence of the self-diffusion coefficient. For both investigated systems, the concentration dependence of the self-diffusion coefficient can quantitatively be described with the model assumptions of a concentrated lattice gas.