Mobile oxygen vacancies

The latter is exemplified by yttria-stabilised zirconia where the acceptor doping is compensated by mobile oxygen vacancies that lay ground for this material as a solid electrolyte ... 

As an illustration, it will be assumed that the predominating defects are doubly charged oxygen vacancies and that the cation dopants have one effective negative charge, e.g. MlJj. The associated complex between v" and MlJj can be written NqMI Y, and compared to the relatively mobile oxygen vacancy, this defect can be considered as immobile as the dopant ions themselves. 

Kroger-Vink notation [4]), the mobile oxygen vacancy concentration, expressed as a site fraction, ao, is the distance between equivalent sites, Vq is a characteristic lattice frequency, and vq exp y " is the jump frequency for the migrating ion. Hy and are the enthalpy and entropy of the migrating ion. 

Doped ceria has been suggested as an alternative electrolyte for low temperature SOFCs [6, 31, 32]. Reviews on the electrical conductivity and conduction mechanism in ceria-based electrolytes have been presented by Mogensen et al. [33] and Steele [34], Ceria possesses the same fluorite structure as the stabilised zirconia. Mobile oxygen vacancies are introduced by substituting Ce " with trivalent rare earth ions as shown in Eq. (1). The conductivity of doped ceria systems depends on the kind of dopant and its concentration. A typical dopant concentration dependence of the electrical conductivity in the (Ce02)i -x(Sm203)x system as reported by Yahiro etal. [3 5] is shown in Figure 4.9. 

Fig. 5.110 Grain boundaries as double Schottky contacts (schematic). The concentration profile of the mobile oxygen vacancies is similar to that of [h j but, on account of Eq. (5.215), [Vq] falls off more rapidly (double charge).

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