Imperfection in Solids

Several small crystals together form solids. However, during crystallisation, imperfections in solids may appear because of the moderate or fast rate of crystal formation. Imperfections, in general, can be defined as irregularities that occur unknowingly in the arrangements of the constituents particles. Today, in this article, we will discuss the study material on imperfection in solids.

Imperfections in Solids 

During crystallisation, any defects in solids occur because of the moderate or fast rate of crystal formation. Based on the irregular arrangement, the defect may either be a line or point defect. When an irregularity occurs around any atom or point in an ideal crystal, it is a deviation point defect. Similarly, if there are arrangement deviations in the entire row of lattice points of crystals, it is referred to as a line defect. 

Types of Imperfections in Solids 

According to the third law of thermodynamics, at a fixed temperature, we obtain perfect crystalline substances without any defects; however, if the temperature increases, imperfections in solids might occur. These imperfections are classified into the following types- 

• Stoichiometric defects
• Non-Stoichiometric defects
• Impurity Defects

Let’s discuss each of these in detail- 

• Stoichiometric defects

The stoichiometric of a compound is exactly the same as represented by its chemical formula. This is termed an intrinsic or thermodynamic defect. Further, it is classified into two parts-

• Vacancy Defect: It can be defined as the vacant sites in the lattice. 
• Interstitial Defect: Its constituents particles travel to the lattice’s interstitial site.

Both interstitial and vacancy defects are mostly for non-ionic solids. The iconic solids that maintain the neutrality of the crystal are represented by Schottky and Frenkel defects. 

• Schottky Defects 

Schottky defects generally occur because of the missing of a similar number of anions and cations from the lattice site. The Schottky defects are most likely to take place in ionic compounds where the ratio of the radius of cation and anion [r+/r–] is not below the unity. 

This is mostly like a vacancy defect where the same number of anions and cations are missing but maintain crystal neutrality. The density of these crystals decreases. Since the ions are present, they can conduct a small amount of electricity. Common examples: NaCl , CsCl , KCl , AgBr.

• Frenkel Defects 

In Frenkel defects, several cations are missing and capture the lattice’s interstitial site. Substances that have low coordination numbers show this type of defect. It is a kind of interstitial or dislocation defect. Since there is no ion movement outside the crystal, the density of these crystals does not change. This defect is mostly seen in ionic compounds whose radius ratio [r+/r–] is less. Common example: ZnS , AgBr , AgI , AgCl. 

• Impurity defects 

NaCl contains a tiny amount of SrCl2 in the molten state in the form of a crystal’s impurity. Because of the presence of SrCl2, a certain site of Na+ ion is captured by Sr2+. To maintain crystal neutrality, one Sr2+ replaces two ions of Na+, and the remaining one site of Na+ will be vacant. Common example: CdCl2 and AgCl. 

Non- stoichiometric Defects

The d block elements of the periodic table showcase the non-stoichiometric defects. These are further classified as the metal excess defects that are listed below- 

Metal excess defect because of the anionic vacancy

In case a compound has excess metal ions, the anion is not present, leading to the formation of a hole at the lattice site. This kind of imperfection is shown by alkali metal halides such as NaCl, KCl and LiCl. When alkali metal halide is heated in the vapour atmosphere of the alkali metal, it leads to the formation of anion vacancies. It is further diffused to the crystal surface and blends with newly generated metal cations. These F-centres give several colours such as NaCl gives a yellow colour, HCl gives pink colour and KCl gives a violet colour. 

Metal excess defect because of the presence of interstitial cation

If an excess positive ion is present in the interstitial site, ZnO shows the defects. If heated, the ZnO loses oxygen reversibly. On heating, it changes colour to yellow as oxygen is not present. 

Metal Deficiency Defect

In the metal deficiency defect, certain cations are missing from the lattice site and to maintain its electrical neutrality, the remaining cation tends to increase its valency. Common examples are FeO, whose composition can range from Fe0.93O to Fe0.96O. In the FeO crystal, some Fe2+ cations are missing. 

Conclusion 

As discussed in the article, imperfections in solid is a crucial topic of Chemistry. Here, we discussed imperfections in solid detail. Along with this, we also talked about types of imperfection in solids in length. This imperfection in solid study material must have helped to attain a greater understanding of imperfection in solid along with other related topics.