Dependence of self-diffusivity on size of impurity atoms in a face-centred cubic solid: existence of an anomalous maximum

Abstract

Molecular dynamics simulation of a solid solution consisting of smaller impurity or solute atoms within a face-centred cubic lattice of larger solvent atoms is reported at a fixed concentration. Dependence of self-diffusivity of the solute atom on the diameter of the solute has been obtained. The self-diffusivity of the solute exhibits an anomalous maximum. This surprising result is explained in terms of the mutual cancellation of forces when the size of the impurity atom is comparable to the neck diameter present in the face-centred lattice. As interactions among different solids can vary widely, here we report studies employing the van der Waals interactions, which are most ubiquitous of all. Our results suggest that the only condition for the existence of a maximum is the presence of these dispersions or van der Waals interactions. Apart from the fact that these results suggest the existence of a size-dependent diffusivity maximum, they unambiguously indicate that a larger solute does not necessarily diffuse more slowly than a smaller sized solute. These are of significance in metallurgy as well as materials science. The results suggest that there will be situations where heterodiffusion of smaller impurity atoms such as H, B, C, N and O present in interstitials in a typical solid of transition or other elements of larger size can exhibit surprisingly large self-diffusivities under suitable conditions.