Shaillee Kaushal is teaching live on Unacademy Plus
XII -01 Solid State XI
Imperfection in Solids ideal crystal ideal crystal is that is which all the lattice points are same across the whole crystal. Any movement of lattice point from its original position gives rise to an imperfect crystal or non-ideal crystal. Perfect or ideal crystals exist only at absolute zero or zero kelvin because at this temperature the entropy is zero and the constituent particles have no randomness
Electronic Imperfections These imperfections arise due to movement of electrons. & Above zero Kelvin the electrons start moving by occupying higher energy states. These free electrons are responsible for conduction of electricity The bonds from which the electrons have been removed on heating become electron deficient and these are referred as holes. & The holes also help in conducting electricity. & Electrons and holes are represented by 'e', 'h' and their concentrations as 'n' and p * For example in Silicone and Germanium the number of free electrons and moles will be same.
Atomic Imperfections Vacancy defects When some of the constituent particles are missing from their normal position, vacancies or holes are created These type of defects Interstitial defects: In this defect the constituent particles are missing from their sites and enter into interstitial sites. Sometimes the constituent particles enter into interstitial sites thereby increasing the density enerally lower down the ensity. This defect makes the crystal conduct electricity
Defects in ionic comounds: Defects in stoichiometric crystals: The crystals in which cations and anions are in the same ratio as indicated by their molecular formulae are called stoichiometric crystals. Point defects do not change the stoichiometry of the crystal These defects are also called intrinsic or thermodynamic defect
Schottky defect > This defect was proposed by Schottky. > In this defect equal number of cations and anions are 9099 electrically neutral OIt is observed in those ionic crystals which have high 000 missing from their lattice sites such that the crystal is 0800060| This is a type of vacancy defect ! cordination number and the size of cation and anion are almost same. Schottky defects Examples: NaCI, KCI, KBr, CsCl, AgBr etc. Due to Schottky defect the density of the crystal decreases Lattice energy and stability decreases Entropy increases.
Frenkel defect .This defect was dicovered by Frenkel . This defect arises due to the movement of cations from their lattice points to the interstitial sites. This defect creates vacancy defect at the original site and interstitial defect at new site. . This is also called dislocation defect. . This defect occurs in those ions which have low co-ordination number and the radius of cation is smaller than the anions. .This defect is generally formed in transition metal halides. like AgCI, AgBr, Agl, ZnS.