Notation for charged defects

In an ionic compound the atoms are charged, and the cations and anions may be assigned a definite integer valence (formal oxidation number). This is in fact a fully valid model in defect chemistry, as will be argued in a discussion on the influence of the varying degree of ionicity, or distribution of valence electrons, in a later paragraph. 

In the following, oxygen ions on regular sites in an oxide will be considered to have a formal valence of -2. The cations have a positive charge so as to make the sum of aU positive and negative charges in the compound equal to zero. 

Similarly, cation vacancies may be neutral or have negative effective charges. To illustrate this let us remove a metal atom from the oxide MO and create a vacant metal ion site. The M + ion picks up two electrons to leave as an atom, leaving behind two positive charges in the form of holes or valence defects. If these are localised at 

Similarly, interstitially dissolved atoms will have zero real and effective charge, but depending on their nature they can be ionised to cations with a real and effective positive charge or to anions with real and effective negative charge. 

Defect electrons and electron holes that are free to move in the oxide have effective negative and positive charges, respectively. They are written e and h . If the electron, for instance, is associated with a cation on a regular site - and may as such be considered a valence defect - the defect may be written.  

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