Non-Random Diffusion in Ionic Crystals
Non-random diffusion can result both from the presence of atonic driving forces and from the motion of defects in a crystal. Defect-related non-random effects appear in two different ways in the kinetic diffusion equations, as correlation effects and as defect-wind effects. In the present paper, the origin and magnitude of these effects during diffusion in a driving force are discussed. Kinetic expressions for the drift velocity <vF> are derived from expressions for the effective frequencies of independent atom jumps and are related to the tracer diffusion coefficient D*. For impurity diffusion in an electric field, deviations from the Nernst-Einstein relation result from defect-wind effects. Recently developed equations for the ionic-impurity drift-mobility when diffusion occurs via divacancies moving on one of the sub-lattices in the NaCl structure are summarized. Extensions of the simple equations derived here to more complex situations are discussed.
KeywordsIonic Crystal Impurity Diffusion Jump Frequency Vacancy Mechanism Atom Jump
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