Crystallographic defect

Point Defects

• Point defects occur at or around a single lattice point
• They are not extended in space
• Point defects typically involve a few extra or missing atoms
• Larger defects in an ordered structure are considered dislocation loops
• Point defects in ionic crystals are often called centers, such as vacancies being called luminescence centers or F-centers

Vacancy Defects

• Vacancy defects are lattice sites that are vacant in a perfect crystal
• Neighboring atoms may move to occupy the vacant site, causing the vacancy to move in the opposite direction
• The stability of the crystal structure prevents neighboring atoms from collapsing around the vacancy
• In some materials, neighboring atoms may move away from a vacancy due to attraction from surrounding atoms
• A vacancy (or pair of vacancies in an ionic solid) is sometimes called a Schottky defect

Interstitial Defects

• Interstitial defects are atoms that occupy a site in the crystal structure where there is usually not an atom
• They are generally high energy configurations
• Small atoms, like hydrogen in palladium, can occupy interstices without high energy
• A nearby pair of a vacancy and an interstitial is often called a Frenkel defect or Frenkel pair
• Interstitial defects can affect the transport of atoms through crystals

Substitutional Defects

• Substitutional defects occur when an impurity atom is incorporated at a regular atomic site in the crystal structure
• The substituting atom may have a different oxidation state than the atom it is replacing (aliovalent substitution)
• Substitutional defects can shift the equilibrium position of the substituting atom away from the lattice site
• Off-center ions are substitutional defects where the substituting ion is substantially smaller than the ion it is replacing
• Isovalent substitution occurs when the substituting ion has the same oxidation state as the ion it is replacing

Other Types of Defects

• Antisite defects occur when atoms of different types exchange positions in an ordered alloy or compound
• Topological defects are regions in a crystal where the normal chemical bonding environment is topologically different from the surroundings
• Amorphous solids can contain defects, such as dangling bonds in ideally bonded amorphous silica
• Complexes can form between different kinds of point defects, such as vacancies binding with impurities
• Line defects, such as dislocations and disclinations, also contribute to crystallographic defects

Crystallographic defect Data Sources