What is Crystal Defect?
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.
Type of Crystal Defect
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 (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
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.
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:
- 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.
- Interstitial Defect- A foreign atom occupying the interstitial location causes an interstitial defect.
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
A Frenkel defect, is a close pair of vacancies and interstitial defects. When an ion goes into an interstitial site, it produces a vacancy.
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 (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
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 is shown below
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
A grain boundary is a general planar defect in a polycrystalline solid that divides regions of varied crystallographic orientation (i.e. grains).
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.