Proton conductors classification and conductivity

The distinction between conductors and insulators is often not well-defined but is, to a certain extent, a matter of choice. Conductivity can vary by more than ten orders of magnitude, sometimes over a temperature interval of only a few degrees. The charge carriers can be either electrons (or holes) or ions (or vacancies). The occurrence of ionic conductivity implies diffusion of matter through the solid and/or at its surface, although the distinction between the latter two is not always clear-cut. In a densely packed structure, ionic diffusion is associated with a local perturbation of the structure, such as the presence of defects which facilitate the migration of ions or vacancies. 

The most common defects in a compound AB are, using Kroger s 

The defects can be charged positively ( ), negatively ( ) or they can remain neutral (x), giving rise to electronic defects, e.g. 

In materials containing ionic bonds, the defects (i.e. ions or vacancies) are charged naturally and therefore ionic transport is synonymous with ionic conduction. 

Ionic compounds, such as halides, sulphides, oxides, nitrides and certain polymers, can be divided into three major groups (i) insulators with a residual ionic conductivity lower than cm S where the elec- 

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