Transport properties and Stokes-Einstein relation in a computer-simulated glass-forming Cu33.3Zr66.7melt

Abstract

Molecular dynamics simulation with a modified embedded atom potential was used to study transport properties and the Stokes-Einstein relation of a glass-forming Cu${}_{33.3}$Zr${}_{66.7}$ metallic melt. Upon cooling, at high temperatures, the self-diffusion coefficients of the two species evolve nearly parallel, whereas they diverge below 1600 K. The viscosity as function of temperature is calculated from the Green-Kubo equation. The critical temperature of mode coupling theory ${T}_{\mathrm{c}}$ is found as 1030 K, from both the transport properties and the \ensuremath{\alpha}-relaxation time. It is found that the Stokes-Einstein relation between viscosity and diffusivity breaks down at around 1600 K, far above ${T}_{\mathrm{c}}$ and even above the melting temperature. The temperature dependence of the effective diameter in the Stokes-Einstein relation correlates closely with the first derivative of the ratio of the self-diffusion coefficients of the two components. We propose that the onset of Stokes-Einstein relation breakdown could be predicted quantitatively by the divergence behavior of diffusion coefficients, and the breakdown of Stokes-Einstein relation is ascribed to the sudden increase of the dynamic heterogeneity.