. Explain the following terms with suitable examples:(i) Schottky defect (ii) Frenkel defect

Explain the following terms with suitable examples:

(i) Schottky defect (ii) Frenkel defect (iii) Interstitials and (iv) F-centres







Best Answer

(i) Schottky defect : This defect arises when equal number of cations and anions are missing from the lattiqe. It is a common defect in ionic compounds of high coordination number where both cations and anions are of the same size, e.g.; KCl, NaCl, KBr, etc. Due to this defect density of crystal decreases and ‘it begins to conduct electricity to a smaller extent. 

(ii) Frenkel defect : This defect arises when some of the ions of the lattice occupy interstitial sites leaving lattice sites vacant. This defect is generally found in ionic crystals where anion is much larger in size than the cation. e.g., AgBr, ZnS etc. Due to this defect density does not change, electrical conductivity increases to a small extent and there is no change in overall composition of the crystal. 

(iii) Interstitials : Atoms or ions which occupy normally vacant interstitials (voids) in a crystal are called interstitials. 

(iv) F-centres : These are the anionic sites occupied by unpaired electrons. F-centes impart colour to crystals. The colour results by the excitation of electrons when they absorb energy from the visible light falling on the crystali F-centres are produced when all alkali halide is heated with excess alkali metal. e.g., when NaCl crystals are heated in presence of sodium vapour, yellow colour is produced due to F-centres. 




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