Impurity Defects: Causes, Types and Examples

Impurity defect is defined as the foreign atoms that compress into the interstices or replace some atoms that make up the solid. The defects in ionic compounds due to replacing ions by ions of another compound are called impurity defects. It is one of the point defects. In Impurity defect, the particular entity of a crystalline solid structure consists of trace or minimum amounts of another entity. Impurity defect occurs when foreign atoms differ from the host atoms that exist in the crystal lattice. It arises when foreign atoms exist at the vacant interstitial sites or lattice sites.

Key Terms: Lattice, Ions, Defects, Impurity, Point defects, Atom, Solid, Crystalline solid, Ionic compound, crystal lattice, Ionic solid

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Formation of Vacancy in Impurity Defect through Aliovalent Impurity

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Vacancies are formed by the addition of impurities through aliovalent ions to an ionic solid. The formation of vacancies in impurity defect through aliovalent impurity are mentioned in the below points:

• Crystallization of molten NaCl with a small amount of SrCl₂ that results in Sr²⁺ which occupies some of the sites of Na⁺
• Two Na⁺ ions are replaced by each Sr²⁺ 
• The site of one ion is occupied by impurity defects and the other site remains empty.
• The number of cationic vacancies that are produced is equivalent to that of Sr²⁺
• Solid solution of AgCl and CdCl₂

The video below explains this:

Imperfections in Electrical and Magnetic properties of Solids Detailed Video Explanation:

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Types of Impurity Defects

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Impurity defects are divided into two types:

• Interstitial Impurity Defects: Interstitial Impurity defects is a type of point defect that occurs when an impurity atom involves a tetrahedral or octahedral hole between atoms in the lattice. It is generally a small atom that can fit into a tetrahedral or octahedral hole in the metal lattice. For eg. Steel comprises iron with carbon atoms that are added as interstitial Impurities. The addition of one or more semimetals or transition metals can build up the corrosion resistance of steel.
• Substitutional Impurity Defects: Substitutional Impurity defect is another type of point defect that occurs when an impurity atom involves a normal lattice site. It is of the same size as the different atoms that replace one of the atoms that form the host lattice. Substitutional impurities are chemically the same as the substance that comprises the bulk of the sample. They normally have atomic radii within a radius of 15% of the host. For eg: Calcium and Strontium have the same radii and are chemically the same which results in strontium as a usual impurity in crystalline calcium, with randomly occupied Sr atoms normally occupied by Ca.

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Impurity Defects in Stainless Steel

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Impurity defects can be explained with the help of stainless steel. We know that stainless steel is composed of chromium and iron. This is the minimal concentration, but according to the specified requirements, it can be more. It contains elements in trace amounts like nickel, carbon, titanium, molybdenum, etc. The sizes of all the metal atoms like nickel, carbon, titanium, molybdenum to some extent are similar to each other though not the same. Due to this similarity, these atoms can form rigid structures in a crystal lattice. But carbon is the one element that is the “odd one out”.

Carbon belongs to the fourteenth group and second period of the periodic table. Therefore it is by nature a metalloid. In its period, carbon is the smallest because it is the uppermost element, and it even needs a d-orbital. The size of this atom fits perfectly the voids that are created by the iron element. In interstitial areas, the carbon atoms remain. It is said to be an impurity due to the irregularity of its distribution and so it disturbs both the long and short-range order of the whole structure of crystalline.

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Things to Remember

• Impurity defect is defined as the foreign atoms that compress into the interstices or replace some atoms that make up the solid.
• In Impurity defect, the particular entity of a crystalline solid structure consists of trace or minimum amounts of another entity.
• Impurity defect occurs when foreign atoms differ from the host atoms that exist in the crystal lattice.
• Impurity defects are essential in the electrical behavior of semiconductors.
• Impurity defects are divided into two types- Interstitial Impurity Defects and Substitutional Impurity Defects.

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