Imperfections or Defects in a Solid

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.

When we talk about crystalline solids, we are saying they have got an exact shape. They have a long time arrangement of their molecules. This means that they've repeated the association of a unit cellular to form an area lattice shape. So the arrangement of molecules, ultimately, will stay the same. Any irregularity within the sample of crystal association in solids is referred to as imperfection in solids. The prevalence of defects takes place whilst the technique of formation of crystals takes place. it can arise at a completely fast or at an intermediate rate. It happens because particles do no longer get sufficient time to set up themselves in a regular pattern.

Reasons of Crystal defect

Defects arise in crystals because of the subsequent factors:

• Vacancies in the lattice

• Dislocation of a particle in the lattice

• Nonstoichiometric proportions of the ions four.

•  Impurities within the lattice

Point Defects: while the deviation occurs around an atom/particle it's far from a point defect. It can be due to displacement, an extra particle or a missing particle.

Line Defects: when there's an abnormality in the arrangement of an entire row, then it is a line defect.

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.

Types of Stoichiometric defect:

Vacancy defect: while the atoms no longer exist at their lattice sites, then there's a vacant lattice site and it does create the “vacancy defect.” Because of this, the substance’s density reduces.

Interstitial defect: Interstitial defect is a defect wherein a molecule or an atom occupies intermolecular spaces in the crystal. In this very illness, the substance's density increases.In particular, the non-ionic compound shows interstitial and emptiness defects; while the ionic compound depicts the same in Schottky and Frenkel defect.

A non-ionic compound primarily represents interstitial and vacancy defects. An ionic compound represents the 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.

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.

• 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.

Impurity Defect:

This defect may be without difficulty understood through the example of molten NaCl being crystallized in conjunction with a compound of SrCl₂. in this, Sr²⁺ ions replace 2 Na+ ions and live in the area of single Na+. in this way, the lattice area of 1 Na+ is empty and it generates the impurity defect.

Non-Stoichiometric Defect:

In non-stoichiometric defects, the ratio of anions and cations is disturbed due to either the addition or elimination of ions.

Forms of Non-Stoichiometric defect:

• Metallic Deficiency defect

• Metallic excess defect

Metal deficiency defect: In this, the solids have much less variety of metals relative to the defined Stoichiometric share.

Metal excess defect: There are two forms of metal excess defect:

Metallic extra defect because of anionic vacancies: This occurs because of the absence of anions from its original lattice site in crystals. Therefore, as opposed to anions, electrons occupy their role in steel excess defect because of the presence of greater cations. 

Excess metal due to the presence of additional cation: Interstitial sites are spaces within the crystal lattice that are free of cations or anions. But the presence of additional cations in the solid ionic region leads to the addition of a few cations that reside in the binding sites and lead to the passage of iron.

For example, Zinc oxide when heated turns yellow from white due to the loss of oxygen leading to excess zinc in the lattice.

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.