Diffusion mechanisms in under-cooled binary liquids

In an amorphous solid two distinct types of jump processes have been observed: collective jumps where a single atom moves only a fraction of the interatomic distance, and jumps where single atoms move over interatomic distances. Upon cooling of a liquid to the glass transition temperature diffusion changes from viscous flow at high temperatures to diffusion via these thermally activated jumps in the amorphous solid. In an Arrhenius representation the self-diffusion constant is strongly curved downwards. We present results of computer simulations of undercooled liquid Zr67Cu33 at temperatures down to the glass transition. Analyzing the atomic displacements two distinct processes contributing to diffusion can be distinguished, resembling the two types of jumps in the solid. Additionally to a more flow-like motion especially the Cu-atoms jump over nearest neighbor distances. The latter process dominates, at least for Cu, at temperatures sufficiently above the glass transition. With decreasing temperature the...