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Stoichiometric Defects
The compounds in which the number of positive and negative ions are exactly in the ratios indicated by their chemical formulae are called stoichiometric compounds. The defects do not disturb the stoichiometry (the ratio of numbers of positive and negative ions) are called stoichiometric defects. These are of following types,
(a) Interstitial defect: This type of defect is caused due to the presence of ions in the normally vacant interstitial sites in the crystals.
(b) Schottky defect: This type of defect when equal number of cations and anions are missing from their lattice sites so that the electrical neutrality is maintained. This type of defect occurs in highly ionic compounds which have high co-ordination number and cations and anions of similar sizes.
Example: NaCl, KCl, CsCl and KBr etc.
(c) Frenkel defect: This type of defect arises when an ion is missing from its lattice site and occupies an interstitial position. The crystal as a whole remains electrically neutral because the number of anions and cations remain same. Since cations are usually smaller than anions, they occupy interstitial sites. This type of defect occurs in the compounds which have low co-ordination number and cations and anions of different sizes.
Example: ZnS, AgCl and AgI etc.
Frenkel defect are not found in pure alkali metal halides because the cations due to larger size cannot get into the interstitial sites. In AgBr both Schottky and Frenkel defects occur simultaneously.

Imperfections in Solids | Chemistry Class 12 - NEET

Non-Stoichiometric Defects:
Nonstoichiometric inorganic solids contain the constituent elements in a non-stoichiometric ratio due to defects in their crystal structures.
These defects are of two types: 
(i) metal excess defect and
(ii) metal deficiency defect.

(i) Metal Excess Defect.
Imperfections in Solids | Chemistry Class 12 - NEET
⇒ Metal excess defect due to anionic vacancies.
⇒ This type of defect is exhibited by alkali halides like NaCl and KCl.
⇒ Application of heat to NaCl in an atmosphere of sodium vapour results in deposition of the sodium atoms on the surface of the crystal.
⇒ The sodium atoms lose electron to form Na+.
⇒ The Cl ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl.

Imperfections in Solids | Chemistry Class 12 - NEET
⇒ The released electrons diffuse into the crystal and occupy anionic sites.
⇒ The anionic sites occupied by unpaired electrons are called F-centres that imparts yellow colour to the crystals of NaCl due to the excitation of the electrons on absorption of energy from the visible light falling on the crystals.
⇒ Excess of lithium makes LiCl crystals pink and excess of potassium makes KCl crystals violet.
⇒ Metal excess defect due to the presence of extra cations at interstitial sites:
⇒ Heating Zinc oxide that exists in white colour at room temperature loses oxygen and turns yellow.

Imperfections in Solids | Chemistry Class 12 - NEET⇒ This results in excess of zinc in the crystal and its formula becomes Zn1+xO.
⇒ The excess Zn2+ ions move to interstitial sites and the electrons to neighbouring interstitial sites.

Metal Deficiency Defect:
⇒ Many solids possess less amount of the metal as compared to the stoichiometric proportion.
⇒ For example, FeO is mostly found with a composition ranging from Fe93O to Fe0.96O.
 In crystals of FeO some Fe2+ cations are missing and the loss of positive charge is made up by the presence of required number of Fe3+.

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FAQs on Imperfections in Solids - Chemistry Class 12 - NEET

1. What are some common imperfections in solids?
Ans. Some common imperfections in solids include point defects (such as vacancies, interstitials, and impurities), line defects (such as dislocations), and planar defects (such as grain boundaries and stacking faults).
2. How do point defects affect the properties of solids?
Ans. Point defects can affect the properties of solids in various ways. For example, vacancies can reduce the density and mechanical strength of a material, while impurities can alter its electrical conductivity or optical properties. Interstitials can also affect the mechanical properties and diffusion behavior of materials.
3. What are dislocations and how do they impact the mechanical properties of solids?
Ans. Dislocations are line defects in the crystal structure of a solid. They can be classified as edge dislocations, screw dislocations, or mixed dislocations. Dislocations can significantly affect the mechanical properties of solids, as they can act as obstacles to the movement of dislocations, influence the material's plastic deformation behavior, and affect its strength and ductility.
4. How do grain boundaries affect the properties of polycrystalline materials?
Ans. Grain boundaries are planar defects that separate individual crystal grains in polycrystalline materials. They can affect the mechanical, electrical, and thermal properties of the material. Grain boundaries can hinder the movement of dislocations, influence the material's strength and ductility, and affect its electrical conductivity and thermal conductivity.
5. How can imperfections in solids be controlled or manipulated in materials engineering?
Ans. Imperfections in solids can be controlled or manipulated through various techniques in materials engineering. For example, impurities can be intentionally added to alter the properties of a material. The growth conditions and processing techniques can also be adjusted to control the formation and distribution of defects. Additionally, heat treatment and mechanical deformation can be used to modify the defects in a material.
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