Origin of the diffusion in a solid

Consider a component in solution in a phase. At a given moment, for various reasons, the phase may not be uniform, for example in eomposition, and thus, the chemical potential of our component may not have the same value at any point of the phase, without however having discontinuity, which would point out the presence of a second phase and a ehange of conqronent. Under these conditions, we can show that if the phase is uniform with respect to all other intensive variables (at a particular temperature), it is not stable and it will evolve by movements of atoms or molecules of our component in order to standardize its chemical potential at any point of the phase. 

This affinity is positive if Then the component moves spontaneously 

Consider, for example, a crystallized, binary solid made up of A and B elements. If this solid does not present any point defects, its composition is uniform, and if all other intensive parameters are also uniform, the solid is said to be in internal equilibrium. The diffusion of one of the species A or B can be considered only if the solid presents other structrrre elements than the oires of the ideal crystal, that is, point defects. Indeed, on the contrary there is no gradient of chemical potential, so atoms or molecules of A or B cotrld not diffuse. 

According to the natirre of poirrt defects allowing the movement, people consider various modes of diffusion [ADD 66]. 

A succession of jumps will cause the total process, that is, the diffusion of vacancies in the whole phase. 

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