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

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 T-c of mode coupling theory (MCT) is derived as 930 K from the self-diffusion coefficient D and viscosity eta. The SE relation breaks down around T-SE = 1900 K, which is far above T-c. At temperatures below T-SE, the product of D and eta fluctuates around a constant value, similar to the prediction of MCT near T-c. 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 T-SE, 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(D-1/D-2)/dT to predict T-SE is confirmed. Additionally we propose a viscosity criterion to predict T-SE in the absence of diffusion data. (C) 2016 AIP Publishing LLC.