  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.
