Frenkel defect
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A Frenkel defect, also called Frenkel disorder, is the formation of an interstitial by an atom or ion leaving its lattice or sublattice. Consequentially, a vacancy is formed where the atom/ion used to be. The defect is named after the Soviet physisist Yakov Frenkel
For example, consider a lattice formed by X and M ions. Suppose a M ion leaves the M sublattice, leaving the X sublattice unchanged. The number of interstitials in this case will equal the number of vacancies formed.
This can be illustrated with the example of the sodium chloride crystal structure. The diagrams below are schematic two-dimensional representations.