Interstitial impurity diffusion in metals; the apparent size effect

It is demonstrated that the abnormally fast impurity diffusion in Pb and α-Ti, which is interstitial-dominated, mainly depends on the solubility. With decreasing solubility, the fraction of interstitially dissolved impurities increases. This leads to a drastic increase of the diffusivity. In the case of small solubilities, the solubility depends on the atomic radius of the solute. This effect erroneously suggests a direct correlation between diffusivity and atomic size (size effect).