Diffusion mechanisms in under-cooled binary liquids of Zr67Cu33

In an Arrhenius representation the self-diffusion constant in under-cooled liquids decreases upon cooling to the glass transition temperature. We pres ent results of computer simulations of molten Zr67Cu33 at temperatures down to the glass transition. Analyzing the atomic displacements we observe two processes contributing to diffusion: a more flow-like motion, similar for Cu- and Zr-atoms, and a hopping of atoms to nearest-neighbour distances. Th e latter process dominates, at least for Cu, at the higher temperatures. Wi th decreasing temperature these jumps become increasingly reversible and th eir contribution to long range diffusion diminishes. We find that the jump- rates obey an Arrhenius law and the rapid drop of diffusion results from th e increased reversibility. (C) 1999 Elsevier Science B.V. All rights reserv ed.