Interfacial reactions and concentration differences

Even considering that the reactions at the interfaces may have proceeded to equilibrium, there is nevertheless a vast difference in the numbers of a given type of defect species which exist in the oxide at the two interfaces this difference then constitutes an effective driving force for transport of the defect species through the oxide layer. The concentration difference of a given species can be considered in terms of a difference in the chemical potential of that species across the oxide and it is this difference in chemical potential which may be considered to lead to a transport of that species through the oxide layer. The chemical and electrochemical 

Note in Fig. 4 that there are four primary ionic defect species which we must consider, namely cation interstitials, cation vacancies, anion interstitials, and anion vacancies (denoted by the superscripts ci, cv, ai and av, respectively). In the case of non-simultaneous place exchange, referred to 

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