Defects in Solids

• The term “defect” or “imperfection” is generally used to describe any deviation from the perfect periodic array of atoms in the crystal.

• The properties of some materials are extremely influenced by the presence of imperfections such as mechanical strength, ductility, crystal growth, magnetic hysteresis, dielectric strength, condition in semiconductors, which are termed structure sensitive are greatly affected by the-relatively minor changes in crystal structure caused by defects or imperfections.

• There are some properties of materials such as stiffness, density and electrical conductivity which are termed structure-insensitive, are not affected by the presence of defects in crystals.

• It is important to have knowledge about the types of imperfections that exist and the roles they play in affecting the behavior of materials.

• Crystal imperfections can be classified on the basis of their geometry as

Crystal imperfections classifications

Point Defects in Metals

It is a zero dimension defect, associated with one or two atomic positions.

Vacancies

The simplest of the point defects is a vacancy, or vacant lattice site, one normally occupied from which an atom is missing. All crystalline solids contain vacancies and, in fact, it is not possible to create such a material that is free of these defects, vacant atomic sites in a structure.

vacancies

Self-Interstitials

when an atom occupies an interstitial site where no atom would ordinarily appear, causing an interstitialcy.

Self- interstitial

Point Defects in Ceramics

Vacancies

vacancies exist in ceramics for both cations and anions.

Interstitials

interstitials exist for cations, interstitials are not normally observed for anions because anions are large relative to the interstitial sites.

Point defect in ceramics

Frenkel Defect

To maintain the charge neutrality, a cation vacancy-cation interstitial pair occur together. This is called a Frenkel defect. The cation leaves its normal position and moves to the interstitial site. There is no change in charge because the cation maintains the same positive charge as an interstitial.

Schottky Defect

A cation vacancy–anion vacancy pair known as a Schottky defect. To maintain the charge neutrality, remove one cation and one anion; this creates two vacancies.

Frenkel Defect To maintain the charge neutrality, a cation vacancy-cation interstitial pair occur together. This is called a Frenkel defect. The cation leaves its normal position and moves to the interstitial site. There is no change in charge because the cation maintains the same positive charge as an interstitial. Schottky Defect A cation vacancy–anion vacancy pair known as a Schottky defect. To maintain the charge neutrality, remove one cation and one anion; this creates two vacancies.

Frenkel and Schottky defect