Crystalline defects

The ideal crystal has an infinite 3D repetition of identical units, which may be atoms or molecules.

Real crystals are limited in size, and they have some disorder in stacking which are called defects.

Point Defects:

• A Point Defect involves a single atom change to the normal crystal array.
• There are three major types of point defect: Vacancies, Interstitials and Impurities.
• They may be built-in with the original crystal growth, or activated by heat.
• They may be the result of radiation, or electric current etc, etc.

Vacancies:

A Vacancy is the absence of an atom from a site normally occupied in the lattice.

Interstitials:

An Interstitial is an atom on a non-lattice site.

There needs to be enough room for it, so this type of defect occurs in open covalent structures, or metallic structures with large atoms.

Impurities:

An Impurity is the substitution of a regular lattice atom with an atom that does not normally occupy that site.

The atom may come from within the crystal, (e.g. a Chlorine atom on a Sodium site in a NaCl crystal) or from the addition of impurities.

The concentration of point defects in a crystal is typically between 0.1% and 1% of the atomic sites, however extremely pure materials can now be grown.

The concentrations and movement of point defects in a solid are very important in controlling colour and deformation.

MOISES PINEDA

CI 18694836

CAF BLOG1