Types of Impurity Defects

Symmetrical patterns in crystal arrangement may be called defects or imperfections of solid. Defects happen faster because particles are closely packed in a solid and they do not rearrange themselves. Small crystals are arranged together closely and made solid. Defects of solid are mainly classified into two types- point defect or line defect. Some defects help to maintain the charge of electrons. Crystal defect takes place due to lattice vacancies, lattice impurities. The addition of ions with different oxidation states of chemical impurities is called impurity defects. 

Crystal defects

Absolute crystals are formed only at 0 K temperature, and other crystals are not absolute. Crystal defect can be defined as the withdrawal of complex order and particle’s periodic arrangement in the crystal lattice.

Causes of crystal defect

crystal defect take place due to the following factors:

• Lattice vacancies
• In a lattice, disturbance of particles.
• A non-stoichiometric amount of ions.
• Lattice impurities.

Types of defects

There are two main types of defects in solids:

• Line defect
• Point defect

Line defect

In such defects, the solid expands above the microscopic region in a row of a crystal lattice.

Point defect

In lattice, disturbance of particles as they move from one place to another cause the imperfection or defect in a crystal lattice. There are three types of point defects:

• Stoichiometric defect
• Non- stoichiometric defect
• Impurity defect.

Impurity defect

The addition of ions with different oxidation states of chemical impurities is called impurity defects. This defect can make a vacancy of cations in a lattice. Impurities that can be added to crystals are called doping. Impurities are added in ions or in the presence of ionic solids. 

An n-type semiconductor is formed when doping is with impurities of electrons. A p-type semiconductor is formed when doping with a deflected electron. 

Example- while crystallisation, SrCl2 impurities are added to NaCl. Na+ ions have oxidation state= +1 and Sr2+ ions have oxidation state = +2, Sr2+ binds to the regular site of Na+ . For maintaining electrical neutrality, Sr2+ ions replace two Na+ ions, and other Na+ ion sites are left vacant. 

Types of impurity defects

There are two types of impurity defects, they are:

• Interstitial impurity defect (according to these defects, compounds have tetrahedral or octahedral holes into atoms)
• Substitutional impurity defect (impure atoms require normal size)

Interstitial impurity defect 

In a lattice, when an impurity atom has a tetrahedral or octahedral hole into atoms, then an interstitial impurity defect occurs. It is a type of point defect. It is very small and can be fixed into a tetrahedral or octahedral hole in a lattice.

For example:

• Carbon atoms with steel that contain iron are attached to interstitial impurities. 
• Steel corrosion due to the addition of semi-metals or transition metal. 

Substitutional impurity defect

It is a type of point defect that happens when an impure atom requires a normal lattice site. Its size is the same as atoms that exchange each other’s position. These impurities are also same, chemically. 

For example, carbon and strontium both are similar chemically, which results in usual impurities in both strontium and calcium crystalline. 

Impurity defects in stainless steel

Stainless steel is the most common example of an impurity defect. As we know, stainless steel is jointly made up of iron and chromium. This concentration is less but it can be more if required. It also comprises small amounts of elements like carbon, nickel, titanium, molybdenum. These metals may be the same size. All elements show the similarity of the same structure and can make a rigid structure in a lattice of crystal except Carbon. Carbon does not form structure in a crystal lattice because it’s a metalloid and the smallest. 

Conclusion

Microscopy is used to study defects in solids. In the case of some defects, the density of a solid is increased, in some cases, it decreases, and in certain solids, it remains unaffected. Some defects help to maintain the charge of electrons and some variation in cations and anions. In the above article, we learned about impurity defects and their types. Mainly defects of solid are of two types- Line defect and point defect and impurity defect is a type of point defect. These various types explain features of defects and the reason behind the imperfection of solid. Impurity defect is the type of point defect, and hence in this, the addition of ions with different oxidation states of chemical impurities is done. And this defect can make a vacancy of cations in a lattice. Impurity defect is further divided into two classes that are interstitial impurity defect and substitutional impurity defect.