• Ideal crystals may have perfectpositional andorientation order with respect togeometrical entities andphysical properties.
• In (defining) real crystals some of these strict requirements may be relaxed:
  • The order considered may be only with respect to the geometrical entity
  • The positional order may be in the average sense
  • The orientational order may be in the average sense
  • Real crystals may contain other defects

Point defects in crystals:

Point defects are defects that occur only at or around a single lattice point. They are not extended in space in any dimension.

Larger defects in an ordered structure are usually considered dislocation loops.

Vacancy defects are lattice sites which would be occupied in a perfect crystal, but are vacant. If a neighbouring atom moves to occupy the vacant site, the vacancy moves in the opposite direction to the site which used to be occupied by the moving atom. The stability of the surrounding crystal structure guarantees that the neighbouring atoms will not simply collapse around the vacancy. In some materials, neighbouring atoms actually move away from a vacancy, because they experience attraction from atoms in the surroundings

Interstitial defects are atoms that occupy a site in the crystal structure at which there is usually not an atom. They are generally high energy configurations. Small atoms in some crystals can occupy interstices without high energy, such as hydrogen in palladium.

A nearby pair of a vacancy and an interstitial is often called a Frenkel defect or Frenkel pair. This is caused when an ion moves into an interstitial site and creates a vacancy.

Antisite defects occur in an ordered alloy or compound when atoms of different type exchange positions.

Topological defects are regions in a crystal where the normal chemical bonding environment is topologically different from the surroundings.

• Complexes can form between different kinds of point defects.
• For example, if a vacancy encounters an impurity, the two may bind together if the impurity is too large for the lattice.

Interstitials can form 'split interstitial' or 'dumbbell' structures where two atoms effectively share an atomic site, resulting in neither atom actually occupying the site.