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A Stone-Wales defect is a point defect that specifically occurs between two π-bonded carbon atoms, such as those graphene or carbon nanotubes. Such defects alter the typical hexagonal structure that carbon forms, leading to two seven sided carbon configurations and two carbon pentagons instead.
Image sources: ( 1 ) ( 2 )
Point defects ( 2 ) are often considered to be zero-dimensional defects at specific points within the crystal lattice of a material. These defects typically only involve a few atoms at most.
There are four main types of point defects (vacancies, self-interstitials, interstitials, and substitional atoms), which can be divided into intrinsic point defects (related to atoms within a pure material) or extrinsic point defects (related to foreign or impurity atoms within a lattice). However, these are not the only four point defects possible - point defects of a sort can even occur within amorphous solids.
Image sources: ( 1 ) ( 3 )
Substitutional defects ( 2 ) are point defects in which an impurity atom takes the place of a native atom within the crystal lattice. Semiconductors often intentionally add substitional defects through doping, such as adding boron or phosphorous to silicon to create an n- or p-type semiconductor, and certain alloys include extraneous elements to create substitional defects for solution hardening purposes.
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A vacancy ( 2 ) is a type of point defect in a crystal structure, whereby an atom is missing from the lattice. Vacancies are naturally occurring in all crystalline materials, and increase in number as the temperature of a material increases.
The presence of vacancies in a material allows for solid state diffusion of the atoms to occur, and the rate of diffusion for a crystal lattice is largely dependent upon the number of vacancies. Vacancies also distort the crystal lattice.
Image 1.
Schottky defects ( 2 ) are a type of vacancy point defects that occur in ionic compounds. They consist of grouped vacancies, that together keep the charge of the compound neutral. For example, a missing Cl- in the NaCl structure would make the material more positive as a result, but if it is paired with a Na+ vacancy, the resulting defect does not create a charge imbalance in the material.
These types of defects occur most often in highly ionic, highly coordinated compounds that have atoms of similar sizes, such as salts like NaCl or AgBr, or oxides with the Fluorite structure such as CeO2 or UO2.
A Frenkel defect ( 2 ) is point defect wherein a single atom creates both a vacancy and an interstitial by moving from a lattice site to an interstitial site within a crystal. It is a neutral defect that does not effect the number of atoms in a crystal, thereby allowing the density to remain the same as well.
Frenkel defects are most common in ionic solids that have large size differences between the anions and cations, such as AgBr or AgI.
Image 2.
An antisite defect ( 2 ) is a type of point defect in which atoms in an alloy switch positions (an A atom on a B site, and a B atom on an A site). It is also known as an atom interchange, and is common in alloys with similarly sized elements, such as copper-gold-silver alloys and semiconducting compounds such as GaAs.
Image 1.
Interstitial defects ( 2 ) are point defects that deal with atoms that fill otherwise unoccupied sites in a crystal lattice. These defects can be self-interstitial, wherein an atom of the original material is situated in the wrong position, or they can be impurities, wherein an atom of another element is inserted into the lattice.
Self-interstitials are often split interstitials, or dumbbell interstitials, wherein two atoms occupy the same lattice site, each of them slightly shifted from exact position. The upper right image above depicts a split interstitial.
If an interstitial of a foreign atom is introduced, it is usually called an impurity, but can also be classified as a solute, if the introduction of the interstitial was intentional. Interstitial alloys/compounds are fairly common, and are types of solid solutions which include low carbon steels. Creating interstitials is also a method of solution hardening a material.
Image sources: ( 1 ) ( 3 )
