Crystallographic defect

A crystallographic defect is any part of a crystal that is different from a theoretical "perfect" crystal structure. Defects can be caused by a different chemical element (an impurity) or by changes to the chemical bonds of the structure.

Even though the word "defect" has the common subtext of being a problem or failure, crystallographic defects can give crystals many useful properties. Defect engineering is the field of materials science that deals with creating defects in crystals that give them properties people want. Colouring synthetic gemstones and semiconductor doping are examples of defect engineering.

Types of defect

Point defects

Images of a molybdenum disulfide crystal using electron microscopy with point defects circled. The top image shows antisites (Mo_S or Mo_S2) and the bottom image shows vacancies (v_S).

A point defect is a defect that happens at one location in the crystal, and does not change the structure further away from that point. There are four main types of point defect, and a few more specific subtypes.

• Substitutional defects have a different atom or ion replacing the normal one.
  • Antisites are a type of substitutional defect where an atom that is normally part of the crystal is found at a site that normally has an atom of a different element.
• Vacancies have a missing atom or ion.
  • A Schottky defect is a group of vacancies where the missing atoms match the chemical formula of the compound.
• Interstitial defects have an extra atom or ion in what would normally be an empty interstitial site.
  • A Frankel defect is an interstitial defect where an atom moves from its normal site into an interstitial site, leaving behind a vacancy.
• Topological defects have a different chemical structure from the rest of the compound.

Line defects

Cross-section and layers of a crystal with an edge dislocation.

A line defect is a defect that follows a line through the whole structure. The most important type of line defect is a dislocation, where the layers of the crystal are moved relative to the line. The amount and direction of movement makes a vector called the Burgers vector.

• An edge dislocation is a movement perpendicular to the line, pushing atoms apart or together. This is sometimes compared to a stack of paper containing a half sheet: away from the edge of the sheet, the paper stacks evenly, but at the edge, two whole sheets are pushed apart to make room for the half sheet.
• A screw dislocation is movement parallel to the line. This makes the chemical bonds shift so that instead of forming flat loops around the line, following the bonds makes a shape like a screw or spiral staircase.
• A mixed dislocation is a mixture of an edge and screw dislocation, with some movement both parallel and perpendicular to the line.

Other types of linear defects exist. A disclination is a type of linear defect where the atoms along the line have a different coordination number from the rest of the crystal, causing the angles of the chemical bonds to change. A disclination is said to be negative if the angles are smaller than the main structure (higher coordination number), or positive if the angles are larger (lower coordination number). In most solid crystals, disclinations are found in pairs of equal sign.

Plane defects

A plane defect is a defect that follows a two-dimensional plane. In three-dimensional materials, the sides of a crystal are plane defects. As well as the edges, many types of plane defect are found inside crystals:

• Grain boundary defects have the same crystal on each side, but rotated so that the crystal lattice does not line up. These boundaries form during crystallisation of polycrystals and separate crystal grains from each other.
• Crystal twinning is caused by a plane defect called a twin boundary. In this kind of defect, two crystals with similar lattices but opposite directions share a layer. For example, a hexagonal close-packed structure has a three-layered structure called ABC. A normal HCP crystal would have the layers ABCABCABCAB, but one with a twin boundary would have ABCABCBACBA.