The Atomistics of Interdiffusion

Diffusion in crystals means site-exchange of different SE s. If the SE s are chemically different, as in binary or multicomponent inhomogeneous systems, the concentration equalization is called chemical (inter-)diffusion. 

In a similar way we find jp p+]- Note that vy/p x is the (activated) jump frequency for the exchange of i with other component atoms (e.g., B, if / = A). Let us express Eqn. (5.101) in terms of volume concentrations and assume that the concentration differences between adjacent planes are small enough so that vp Wp = vp/p, is a valid assumption. If d is the distance between the planes, then 

Equation (5.104) expresses the detailed balance in the closed lattice system. If the crossing from one plane to the next is thermally activated, so that v= v0-e i p)/kT, Eqn. (5.104) takes on the following form 

This equation tells us that under the conditions implicit in Eqn. (5.104), the steady state is in fact the (dynamic) equilibrium state in which the thermodynamic functions p(i) = E(i) + kT- n (N(i)/1 -N(i)) are constant in space, that is, Vp(i) = 0. 

Net fluxes occur at off-equilibrium which are (to a first order) driven, by V/u(i). This statement is analogous to the identification of the driving force X,- = Vpij in irreversible component transport (see Section 4.2.1). 

---