Charged Interfaces, Double Layers, and Debye Lengths

6 Charged Interfaces, Double Layers, and Debye Lengths 

This asymmetry in force in turn induces the ion to migrate one way or another. If it is further assumed, again for the sake of simplicity, that in this case the O ions are immobile and that the driving force is such as to induce the M ions to migrate to the surface, then it follows that the interface or surface will now be positively charged with respect to the bulk — a charge that has to be balanced by a negative one in the bulk. For a pure MO compound, this is 

Most oxides contain impurities, which in an attempt to reduce the strain energy of the system tend to migrate to the interfaces, grain boundaries, and surfaces. It is usually the segregation of these impurities that is responsible for the surface charge. This charge is usually compensated, however, with bulk ionic defects. 

For reasons that will become apparent in Chap. 7 (namely, diffusion), it can be shown that the compensating charges to the one at the interface, i.e.. the cation vacancies in this case, are not concentrated in a plane but rather are diffusely distributed in the bulk of the solid, as shown in Fig. 5.5c and 

for example, J. Bockris and A. K. N. Reddy. Modern Electrochemistry. Plenum. New York. 1970. 

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