Non-Fickian current enabling the uphill diffusion of impurities diffusing by the mechanism of mobile impurity-defect pairs

Equations governing the diffusion mediated by the mobile pairs have been derived. In addition to known non-Fickian (n-F) behavior observable at small spatiotemporal scale under homogeneous defect distribution the equations predict another type of n-F diffusion that may take place only in the inhomogeneous case and can be operative at any scale. In the limit of slowly varying inhomogeneity the n-F diffusion current has been identified and shown to be able to induce the uphill diffusion. It has been argued that the latter should be observable in about one third of impurities in FCC metals.