Schottky defect

A Schottky defect is an excitation of the site occupations in a crystal lattice leading to point defects named after Walter H. Schottky. In ionic crystals, this defect forms when oppositely charged ions leave their lattice sites and become incorporated for instance at the surface, creating oppositely charged vacancies. These vacancies are formed in stoichiometric units, to maintain an overall neutral charge in the ionic solid. Schottky defects consist of unoccupied anion and cation sites in a stoichiometric ratio. For a simple ionic crystal of type A−B+, a Schottky defect consists of a single anion vacancy (A) and a single cation vacancy (B), or v + v following Kröger–Vink notation. For a more general crystal with formula AxBy, a Schottky cluster is formed of x vacancies of A and y vacancies of B, thus the overall stoichiometry and charge neutrality are conserved. Conceptually, a Schottky defect is generated if the crystal is expanded by one unit cell, whose a prior empty sites are filled by atoms that diffused out of the interior, thus creating vacancies in the crystal. Schottky defects are observed most frequently when there is a small difference in size between the cations and anions that make up a material. Chemical equations in Kröger–Vink notation for the formation of Schottky defects in TiO2 and BaTiO3. ∅ v + 2 v ∅ v + v + 3 v This can be illustrated schematically with a two-dimensional diagram of a sodium chloride crystal lattice: The vacancies that make up the Schottky defects have opposite charge, thus they experience a mutually attractive Coulomb force. At low temperature, they may form bound clusters. The bound clusters are typically less mobile than the dilute counterparts, as multiple species need to move in a concerted motion for the whole cluster to migrate. This has important implications for numerous functional ceramics used in a wide range of applications, including ion conductors, Solid oxide fuel cells and nuclear fuel.