Ionic space charge formation

Ionic solids (typified by most of the supports in common use) for which the energies of formation of cation and anion vacancies are markedly different seem to possess non-stoicheiometric surfaces containing an excess of the defects most easily formed balanced by space-charge layers within the contiguous bulks. Similarly, the many types of defect possible in semiconductors will be different in concentration, and possibly in kind, in and near the surface. ... 

Implications. These results have an important implication concerning the use of Fourier analysis of DC transients in polymeric materials to extract the frequency-dependence of the dielectric response (12)- In order for the principle of superposition to apply the electric field inside the material being measured must be time- and space-invariant. This critical condition may not be met in polymers which contain mobile ionic impurities or injected electrons. Experimentally, we can fix only the average of the electric field. Moreover, our calculations demonstrate that the bulk field is not constant in either time or space. Thus, the technique of extracting the dielectric response from the Fourier components of the transient response is fundamentally flawed because the contribution due to the formation of ionic and electronic space-charge to the apparent frequency-dependent dielectric response can not generally be separated from the dipole contribution. 

Above 140 K, the situation is quite different. Ionic relaxation, either in the form of Frenkel pair generation adjacent to the impurity or silver ion diffusion from the space-charge layer, provides an alternative decay pathway for the shallowly trapped electron state. The resultant Agj+ is reduced by electron transfer from the impurity trap, a process that has been followed by EPR in particular detail in silver chloride crystals and emulsions. For Pb2 + in AgCl, this ionic decay mechanism has an activation energy of 0.36 + 0.05 eV [85,105], approximately the sum of the formation and diffusion energies for a silver ion interstitial in AgCl [42]. This result supports the Frenkel pair mechanism for the annihilation of the electron shallowly trapped at Pb2+. Similar results have been obtained from EPR studies of Cd2 +-doped AgCl. 

A superlimiting current in an electrolyte is an ionic current large enough to exceed diffusion limitation, which depletes the bulk salt concentration and leads to the formation of extended space charge. 

Additional sources for potential barriers in ionic systems can be driven by intrinsic ionic processes. As first described by Frenkel, the formation of a net surface charge and a compensating space charge layer relates to the energy differences required to bring various ionic species to a surface [13]. Indeed, while ionic soUds are macro-scopicaUy charge-neutral, local variations in both structure and chemistry lead to internal electrostatic potentials and electric fields. Space charge layers are formed... 

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