The Nature and Role of Surface Charge in Ceramics
The recognition by Mott and Littleton1 and Frenkel2, that the energies of formation of cation and anion vacancies in ionic solids are different, led to the conclusion that unequal numbers of these defects can exist in a crystal. In the standard sense of a neutral reference state for the perfect ionic lattice, these defects, as well as interstitials, dislocation jogs, and surface kinks carry net charges. Since surfaces act as sources or sinks for vacancies, the inequality in the number of oppositely charged vacancies would lead to the formation of a surface charge. Compensation for this charge comes from oppositely charged defects residing in a region adjacent to the surface. This region, termed the Debye-Hückel layer, screens the charge on the surface from the bulk of the crystal which is neutral. Lehovic3, Eshelby et al.4 Kliewer5, 6, and Kliewer and Koehler7 extended the theoretical work of Frenkel and included an analysis for impurity-containing crystals. Moreover, since an edge dislocation acts as a source or sink for vacancies, it should also aquire a charge and have a compensating cylindrical region surrounding the dislocation line.
KeywordsSurface Charge Space Charge Region Cation Vacancy Anion Vacancy Kink Site
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