An Explanation of the Differences in Diffusivity of the Components of the Metallic Glass Pd43Cu27Ni10P20

Bartsch et al. [A. Bartsch, K. Raetzke, A. Meyer, and F. Faupel, Phys. Rev. Lett. 104, 195901 (2010)] reported measurements of the diffusivities of different components of the multi-component bulk metallic glass Pd43Cu27Ni10P20. The diffusion of the largest Pd and the smallest P were found to be drastically different. The Stokes-Einstein relation breaks down when considering the P constituent atom, while the relation is obeyed by the Pd atom over 14 orders of magnitude of change in Pd diffusivity. This difference in behavior of Pd and P poses a problem challenging for explanation. With the assist of a recent finding in metallic glasses that the beta-relaxation and the diffusion of the smallest component are closely related processes by Yu et al. [H. B. Yu, K. Samwer, Y. Wu, and W. H. Wang, Phys. Rev. Lett. 109, 095508 (2012)], we use the Coupling Model (CM) to explain the observed difference between P and Pd quantitatively. The same model also explains the correlation between property of the beta-relaxation with fragility found in the family of (CexLa1-x)68Al10Cu20Co2.