The ordinary flux equation for diffusion, which considers the composition gradient as the driving force, was seldom of much use in studying closely stoichiometric phases. Depending on the defect structure it would instead be profitable to use an appropriate function of the activity. Such functions will now be derived and it will be shown how the operating defect mechanism of diffusion could be identified from information on the variation of the activity inside a phase during diffusion. However, it was usually very difficult to measure the activity profile inside a phase. It will be shown how it could be obtained by combining results from several experiments.

Diffusion in Interstitial Compounds with Thermal and Stoichiometric Defects. M.Hillert, L.Höglund, J.Ågren: Journal of Applied Physics, 2005, 98[5], 053511 (6pp)