Crystal Defects: What is Crystal Defect?, Types of Defects

By Mohit Uniyal|Updated : August 3rd, 2022

Crystal Defects control and affect a lot of properties of materials. Electric and thermal conductivity in metals can be strongly reduced by point defects whereas electric conductivity in semiconductors can be controlled by substitution defects. Ionic conductivity means vacancy defects can control the movement of ions. Crystal defects play an important role in the behavior of any material.

Plastic deformation of metals takes place mainly by shearing. The material's lattice planes slide past each other, allowing macroscopic shape change without significantly changing the atom's ordering and arrangements. Plastic deformation in crystalline material is a result of dislocation(type of crystal defects). The crystal defects strongly influence the mechanical strength of the material.

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What are Crystal Defects?

An ideal crystal can be described in terms of a three-dimensionally periodic arrangement of points called lattice and an atom or group of atoms associated with it.

Crystal lattice = Lattice + Motif

where the lattice is a 3D periodic arrangement of points and Motif is an atom or group of atoms. Any deviations from the perfect arrangement of atoms in a crystal are called crystal defects. A crystal defect is a lattice irregularity having one or more of its dimensions on the order of an atomic diameter.

Imperfections have a significant impact on the characteristics of some materials. Because of this, it's critical to understand the different kinds of crystal defects that occur and how they affect how materials behave. For example, alloying significantly affects the mechanical characteristics of pure metals (i.e., when impurity atoms are added)—e.g., sterling silver (92.5% silver-7.5% copper) is much harder and stronger than pure silver.

Types of Crystal Defects

Classification of crystal defects (microscopic defects) is frequently made according to the geometry or dimensionality of the defect. Other macroscopic defects that are far larger than microscopic exist in all solid materials, such as cracks, pores, foreign inclusions, and other phases. Various types of crystal defects which are studied to improve the properties are given below-

  • Point Defects
  • Line Defects
  • Planar or Interfacial Defects

Point Defects

Point defects (type of crystal defects) have atomic dimensions and occur only at or around a single lattice point. They are not stretched in any dimension in space. Different types of point defects in crystals are shown below -

  • Vacancy Defects
  • Interstitial Defects
  • Substitutional Defects
  • Frenkel Defects
  • Schottky Defects

Vacancy Defects

A missing atom at a lattice position causes a vacancy defect. The surrounding crystal structure's stability ensures that the atoms do not just collapse around the void. The vacancy kind of defect can be caused by poor packing during the crystallisation process, or by enhanced thermal vibrations of the atoms caused by high temperatures.

Interstitial Defects

When an atom occupies the interstitial location of the lattice structure, an interstitial defect develops. This interstitial atom might come from the same crystal or another substance. Accordingly, there are two types of interstitial defects:

  1. Self-Interstitial Defect - When an atom from the same crystalline solid occupies an interstitial location instead of its original lattice site, this is known as a self-interstitial defect.
  2. Interstitial Defect- A foreign atom occupying the interstitial location causes an interstitial defect.

Substitutional Defects

Substitutional defects are caused by an impurity in a lattice location. The host atoms are replaced or substituted for by solute or impurity atoms in the substitutional type.

The degree to which the solute dissolves in the solvent depends on several characteristics of the atoms in both. The Hume-Rothery rules are used to express these. Substitutional solid solutions may form according to these rules if the solute and solvent-

  • have a similar radius of atoms (15% or less difference)
  • have the same crystal structure
  • have similar electronegativity
  • have similar valency a solid solution mixes with others to form a new solution

Frenkel Defects

A Frenkel defect, is a close pair of vacancies and interstitial defects. When an ion goes into an interstitial site, it produces a vacancy.

Schottky Defects

Ionic Solids include Schottky defects. The electrical neutrality of ionic compounds must be balanced, nevertheless, to ensure that an equal number of anions and cations are absent. It lessens the substance's density.

Line Defects

Line defects (type of crystal defects) typically span a large number of atoms. Dislocations are line defects that only appear in crystalline materials. A dislocation is a linear or one-dimensional imperfection in which certain atoms are misaligned.

Dislocations are particularly essential in materials science because they contribute to material mechanical strength. The two fundamental forms of dislocations are -

  • Edge dislocation line
  • Screw dislocation

Edge Dislocation

The edge dislocation line, which is defined along the end of the additional half-plane of atoms, is the central focus of edge dislocation. The defect may run in a straight line through the crystal or it may take an irregular path.

Movement of edge dislocation shown below

Screw Dislocation

Screw dislocations can be created by tearing the crystal parallel to the slip direction. A screw dislocation would display a slip pattern like a screw thread if it were followed around an entire circuit.

Planar or Interfacial Defects

Interfacial defects (type of crystal defects) are two-dimensional barriers that generally separate sections of materials with various crystal structures and/or crystallographic orientations. Different types of planar defects are -

  • Grain Boundaries
  • Twinning

Grain Boundaries

A grain boundary is a general planar defect in a polycrystalline solid that divides regions of varied crystallographic orientation (i.e. grains).

Twinning

A twin boundary is a form of grain boundary that has mirror lattice symmetry; that is, atoms on one side of the boundary are in mirror image locations of atoms on the other side. It is correct to refer to the material region between these limitations as a twin.

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FAQs on Crystal Defects

  • Any deviations from the perfect arrangement of atoms in a crystal are called crystal defects. They are classified on the geometry or dimensionality of the defect. Crystal defects can change the properties of a material to a large extent.

  • There are three major types of crystal defects which are mentioned below:

    • Point Defects
    • Line Defects
    • Planar Defects
  • Twin boundaries are types of planar surface defects. The atomic configuration on one side of a twin boundary mirrors the configuration on the other. Twin boundaries occur in pairs such that the orientation change generated by one boundary is restored by the other.

  • Many crystal properties, including strength, electrical conductivity, and hysteresis loss of ferromagnetism, are strongly influenced by crystal defects. As a result, some important features of crystals are affected by defects as well as the nature of the host crystals.

  • Screw dislocation lies parallel to the burger vector and moves perpendicular to the Burgers vector. Screw dislocations can be regarded as the result of shear stress applied to cause distortion.

  • The Frankel defect is caused by an ion being displaced from the crystal lattice. When an atom or cation moves from its initial place in the lattice structure to fill a void while occupying another interstitial position inside the solid crystal, the result is a point defect known as a Frenkel defect. This defect causes vacancy defect at the old site and interstitial defect at the new site.

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