Structural aspects of the Stokes-Einstein relation breakdown in high temperature melts

Dynamical and structural properties of four Al-Fe melts (Al100-xFex, x = 10, 20, 30, and 40) were calculated using molecular dynamics simulation over a wide temperature range. The dynamical results reveal that, a crossover from Arrhenius to non-Arrhenius behavior occurs for α-relaxation time τα and shear viscosity η bordered at Tx ~ 1.2TL (TL the liquidus temperature). At Tx, we also found the breakdown of Stokes-Einstein relation and the rapid enhancement of dynamic heterogeneity. The structural results demonstrate that the fraction of icosahedral cluster increases with decreasing temperature, and the amplitude of increase has a significant shift at Tx. In order to facilitate a uniform description, local five-fold symmetry (LFFS) was selected to describe the development of various kinds of atomic clusters. It is seen that LFFS has an obvious change at Tx, implying a strong coupling between atomic dynamics, SER breakdown and liquid structure change. For the first time, the breakdown of SER is demonstrated to have a structure signature which could be characterized by LFFS. This work would be helpful in understanding the longstanding challenges of the dynamics-structure relationship in liquid metals.