Hydrogen-Diffusion Mechanism in Amorphous ${\mathrm{Pd}}_{85}$${\mathrm{Si}}_{15}$${\mathrm{H}}_{7.5}$: a Neutron-Scattering Study

By means of quasielastic neutron scattering we show that the H-diffusion mechanism in ${\mathrm{Pd}}_{85}$ ${\mathrm{Si}}_{15}$ ${\mathrm{H}}_{7.5}$ is characterized by two well-separated regimes of jump rates, in clear contradiction to present ideas on diffusion in amorphous substances. A semiquantitative interpretation of the data is possible in terms of a two-state diffusion-trapping model. We find no evidence for the hypothesis that the network of diffusive paths in an amorphous substance has a fractal dimension.