Crystal Defects Part -1

Crystal defects

Point defects – it arises due to irregularities of atom around a point 

Line defect – it arise due to irregular writers of atoms in entire row of crystals

 Types of point defects

  • Stoichiometric defect
  • Non Stoichiometric defect
  • Impurity defect

Stoichiometric  defect 

The number of cations and anions are exactly same in ratio as indicated by ideal  chemical composition

Stoichiometric  defect in non ionic compound

a) Vacancy defect

Some of the atoms leave their lattice site and create a vacancy in a crystal is called vacancy defect

The density decreases because mass decreases but volume remains same This effect arises when substance is heated

 

     

b) Interstitial defect 

When some of the constituent particle occupy interstitial site is called interstitial defect

Density increases because mass increases and volume remains same

Stoichiometric defect in ionic compound

Difference between Frenkel and Schottky defect

Schottky defect

Frenkel defect

 

 

Same number of anion and cation are missing from their lattice site

An ion (mostly cation due to small size )missing from their lattice site and occupied

interstitial site in a crystal

Electrical       neutrality      is maintained

Electrical       neutrality      is maintained

Density decrease because mass decrease due to missing of cation and anion from their

Density remains same because no ions is missing

lattice site and volume remains same

 

Compounds exhibit schottky defect must have less difference in size of cation and anion

Large difference in size of cation and anion

Example 

NaCl , KCl , AgBr 

AgI , AgBr , AgCl , ZnS 

 Impurity defect

 If molten NaCl containing a little amount of SrCl2 is crystallised, some of the sites of Na+ ions are occupied by Sr2+. Each Sr2+ replaces two Na+ ions. It occupies the site of one ion and the other site remains vacant. The cationic vacancies thus produced are equal in number to that of Sr2+ ions. Another similar example is the solid solution of CdCl2 and AgCl.