Imperfections or Defects in a Solid

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What are Point Defects in Solids?

Point defects describe the solid imperfections along with the point defect types. The crystalline solids are produced by joining multiple smaller crystals. Various types of defects can be found in crystals after the crystallization process. Point defects account for when the process of crystallization takes place at a faster rate. These defects primarily take place because of the deviation in the arrangement of particles constituting. When the ideal arrangement of solids is distorted around an atom/point in a crystalline solid, it is referred to as a point defect.

Imperfections or Defects present in a crystalline solid can be divided into 4 groups, namely: Line defects, Point defects, Volume defects, Surface defects. The crystal point defects type was first regarded in the ionic crystals, but not in metal crystals, which are much simpler.


Types of Point Defects in Solids

There exist 3 types of point defects, which are listed below:

  • Stoichiometric defect

  • Frenkel defect

  • Schottky defect


Stoichiometric Defect

In this type of point defect, the positive and negative ions (Stoichiometric) ratio, whereas the electrical neutrality of a solid is not disturbed. Few times, it can also be known as thermodynamic or intrinsic defects.

Fundamentally, These Are of Two Types:

  • Vacancy Defect: When an atom is not present at their lattice sites, then that particular lattice site is vacant and forms a vacancy defect. Because of this, the density of a substance decreases.

  • Interstitial Defect: In this type of defect, a molecule or an atom occupies the crystals' intermolecular spaces in this particular defect, the density of the substance increases.

A non-ionic compound primarily represents interstitial and vacancy defects. An ionic compound represents the same in Schottky and Frenkel defect.


Frenkel Defect

In general, in ionic solids, the smaller ion (called cation) moves out of its place and occupies an intermolecular space. In this scenario, a vacancy defect will be created in its original position. The interstitial defect can be experienced in its new position.

  • It is also referred to as a dislocation defect.

  • It takes place when there is a huge difference in anions and cations size.

  • Here, the substance's density remains unchanged.

  • An example is AgCl and ZnS.

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Schottky Defect

  • This type of vacancy defects can be found in the Ionic Solids. However, coming to ionic compounds, we are required to balance the electrical neutrality of the compound. Thus, an equal number of cations and anions will be missing from the compound.

  • Here, the size of the anions and cations are almost the same.

  • This defect reduces the density of the substance.

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  • Impurity Defect: Let us understand the impurity defect with a brief example. If a molten NaCl is crystallized with SrCl2 compound, the Sr2+ ions replace 2 Na+ ions and occupy 1 Na+. In this manner, the lattice site of 1 Na+ is vacant, and it produces an impurity defect.

  • Non-Stoichiometric Defect: In this defect type, the anions and cations ratio is disturbed either due to the adding or removing ions.


Types of Non-Stoichiometric Defect

  • Metal deficiency defect: In this defect, the solids contain fewer metals relative to the specified Stoichiometric proportion.

  • Metal excess defect: There exist 2 types of metal excess defects, as given below:

    • Metal Excess Defects Due to the Anionic Vacancies: This takes place because of the absence of anions from their original lattice site in the crystals. Thus, instead of anions, electrons will occupy their position

    • Metal Excess Defect Due to the Presence of Extra Cations at Their Interstitial Sites: In this type, on heating the compound, it releases an extra cation, where these cations occupy the interstitial sites in the crystals and the same electron count goes to the neighbouring interstitial sites.


Applications of Point Defects

Let us look at and understand various applications of different defects, as listed below.

Application of Interstitial Defect

If an interstitial impurity produces a polar covalent bond to the host atoms, the layers will be prevented from sliding past one another, even when only a less amount of the impurity exists.

For example, because the iron produces the polar covalent bonds to carbon, the strongest steel must contain only about 1 percent carbon by mass to substantially increase its strength.


Application of Defect of Deformation

A flexible and fatigue-resistant alloy, which is composed of nickel and titanium, is given as Flexon. Originally, it was discovered by Metallurgists, who were creating the titanium-based alloys to use in heat shields of the missile. Now, flexon can be used as a corrosion-resistant and durable frame for glasses, among other uses.


Application of Substitutional Defect

The substitutional impurities can be observed in the molecular crystals if the host's impurity structure is similar. They have the main effects on the crystal properties. For example, pure anthracene is an electrical conductor. However, the electron transfer via a molecule is prolonged if the anthracene crystal has minimal tetracene amounts despite their stronger structural similarities.

FAQ (Frequently Asked Questions)

1. Give the defects of Crystal Structure?

Let us look at some of the defects of crystal structures.

  • The 0-dimensional defects can be given as vacancies and/or impurity particles present at different lattice sites present in the crystal.

  • The 1-dimensional defects can be referred to as dislocations or line defects. These dislocation movements strongly influence the materials' mechanical properties.

2. What is the point of imperfection?

Point imperfection is also referred to as Point Defect. Imperfections or Defects present in the crystalline solid can be divided into 4 groups: line defects, point defects, volume defects, and surface defects. Historically, the crystal point defects were first regarded in the ionic crystals, but not in the metal crystals, which were much simpler.

3. Why do the BCC crystals not have the stacking faults?

Introducing a stacking fault

When we imagine how the BCC looks if we have no room left for the stacking fault.

Suppose that Layer A is on top of B. Moreover, the Next layer has no other option but to be exactly on to the top of layer A. And, there is no other position the 3rd layer could be as opposed to the hexagonal/FCC lattice, which differs only by the 3rd layer position. Thus, it is possible to have a stacking error.

4. What are some of the qualities of metals?

Let us look at a few qualities of metals as listed below.

  • Produces the highest melting point

  • Holds at high temperatures

  • Undergoes the catastrophic nature