We discuss the dependence of self- and interdiffusion coefficients on
temperature and composition for two prototypical binary metallic melts, Al-Ni
and Zr-Ni, in molecular-dynamics (MD) computer simulations and the
mode-coupling theory of the glass transition (MCT). Dynamical processes that
are mainly entropic in origin slow down mass transport (as expressed through
self diffusion) in the mixture as compared to the ideal-mixing contribution.
Interdiffusion of chemical species is a competition of slow kinetic modes with
a strong thermodynamic driving force that is caused by non-entropic
interactions. The combination of both dynamic and thermodynamic effects causes
qualitative differences in the concentration dependence of self-diffusion and
interdiffusion coefficients. At high temperatures, the thermodynamic
enhancement of interdiffusion prevails, while at low temperatures, kinetic
effects dominate the concentration dependence, rationalized within MCT as the
approach to its ideal-glass transition temperature Tc. The Darken equation
relating self- and interdiffusion qualitatively reproduces the
concentration-dependence in both Zr-Ni and Al-Ni, but quantitatively, the
kinetic contributions to interdiffusion can be slower than the lower bound
suggested by the Darken equation. As temperature is decreased, the agreement
with Darken's equation improves, due to a strong coupling of all kinetic modes
that is a generic feature predicted by MCT.Comment: 16 pages, 12 figure
