Interstitialcy and the Kick-Out Mechanism

When the diffusing interstitial atoms have a size comparable to that of the lattice atoms, the interstitialcy mechanism for diffusion may take place. In this case the interstitial impurity atom moves into a host lattice site by pushing a neighboring normal atom into the adjacent interstitial site. This process repeats itself when a self-interstitial atom pushes the substitutionally located impurity atom into an interstitial site. 

The kick-out mechanism is rather similar to the interstitialcy mechanism. In this case, a host self-interstitial atom diffuses around the lattice. When it reaches a substitutional impurity atom, the self-interstitial pushes the impurity atom into an adjacent interstitial site. The interstitial impurity then diffuses interstitially until it reverts back to a host lattice site by displacing a host atom. It is experimentally difficult to distinguish the kick-out mechanism from the interstitialcy mechanism. The generally accepted view is that the interstitial impurity atoms may tend to diffuse longer distances before returning to the normal lattice sites for the kick-out mechanism, whereas the impurity atoms tend to diffuse interstitially for a relatively short distance before going into the normal lattice sites for the interstitialcy mechanism. 

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