A simple model for disordered materials was treated by using numerical and analytical approaches. The disorder involved a distribution of site energies (site disorder) and saddle energies (saddle disorder) which had opposite effects upon long-range mass transport. It was found that the continuous-time random walk approach accounted well for the results on site disorder. Its failure in the case of saddle disorder had to be corrected by including correlation effects. The percolation approach appeared to be a convenient framework, for the description of the low-temperature regime from a qualitative point of view, but the presence of continuous disorder prevented the use of universal exponents. The transient dispersive regime was not well explained by current percolation modelling. The present model offered an explanation for the fact that an Arrhenius behavior of the long-range diffusivity was observed experimentally, while the apparent pre-factor was not simply related to the entropy change that was suffered by the elementary jump mechanism.

Y.Limoge, J.L.Bocquet: Materials Science Forum, 1996, 223-224, 175-86