High temperature breakdown of the Stokes-Einstein relation in a computer simulated Cu-Zr melt.

Abstract

Transport properties and the Stokes-Einstein (SE) relation in liquid Cu8Zr3 are studied by molecular dynamics simulation with a modified embedded atom potential. The critical temperature Tc of mode coupling theory (MCT) is derived as 930 K from the self-diffusion coefficient D and viscosity η. The SE relation breaks down around TSE = 1900 K, which is far above Tc. At temperatures below TSE, the product of D and η fluctuates around a constant value, similar to the prediction of MCT near Tc. The influence of the microscopic atomic motion on macroscopic properties is investigated by analyzing the time dependent liquid structure and the self-hole filling process. The self-holes for the two components are preferentially filled by atoms of the same component. The self-hole filling dynamics explains the different breakdown behaviors of the SE relation in Zr-rich liquid CuZr2 compared to Cu-rich Cu8Zr3. At TSE, a kink is found in the temperature dependence of both partial and total coordination numbers for the three atomic pair combinations and of the typical time of self-hole filling. This indicates a strong correlation between liquid structure, atomic dynamics, and the breakdown of SE relation. The previously suggested usefulness of the parameter d(D1/D2)/dT to predict TSE is confirmed. Additionally we propose a viscosity criterion to predict TSE in the absence of diffusion data.