Atomic and Ionic Defects

Relative to the normal situation at the site, the vacancy appears to bear an effective negative charge equivalent to —1. Hence, a vacancy at a sodium ion (Na+) site in NaCl would be written as V N 1. 

The same reasoning indicates that the absence of a negative ion will leave a positive effective charge relative to a normal site occupied by a negative ion. A vacancy at a chloride ion (Cl-) site is positively charged relative to the normal situation prevailing at an anion site in the crystal. Using Eq. (1.1)  

the vacancy has an effective charge of +1, which would be written V, . 

In general, the absence of a negative ion will endow a site with a positive effective charge. Multiple effective positive charges can exist and are written using superscript n . An oxide ion (O2-) vacancy in a crystal of CaO will bear an effective positive charge of 

An effective charge relative to the host lattice is possible with any defect. These are added as superscripts to the appropriate symbol VM, Vx, M , Mx and associated defects such as (VMVX). 

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