Mass Action and Point Defects

The law of mass action may be applied to reactions involving the formation and interaction of lattice defects which meet the criteria discussed in Section 1.1. Equation (3) is applicable when a crystal is in equilibrium with its ambient. At low concentrations of defects, concentrations may be used in place of activities in Eq. (3). In such cases, brackets usually are used to denote concentration, so that Eq. (2) may be written 

The simplest types of native atomic defects are point defects. These may be vacancies (unoccupied normal sites in the lattice), interstitial defects (atoms situated on normally unoccupied interstices), and substitutional defects (one type of atom substituting for another on a normal site). All these defects may be treated as chemical entities in the application of the law of mass action. For example, the formation of Frenkel defects (vacancy plus interstitial) in a lattice of A atoms may be written as follows  

At higher defect concentrations, may differ from unity to a significant degree. In addition, it is necessary to treat lattice sites as chemical entities. In writing the symbol A, the A atom is implicitly assumed to be associated with a normal site. Therefore, formation of a vacancy by removal of an A atom from a crystal by the reaction 

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