4 Marks Questions

Question 14 Marks

What is the difference between Schottky and Frenkel defect ?

Answer

	Frenkel Defect	Schottkey Defect
1.	The positive and negative ions (cations and anions) leaves its lattice position and occupies a vacant interstitial site existing between the cations and anions in the crystal.	The positive and negative ions are completely missing from their lattice positions. The number of cations and anions which are missing is the same.
2.	Density remains the same.	Density of the crystal decreases.
3.	Dielectric constant incre-ases.	Dielectric constant remains the same.
4.	Stability decreases and electrical conductivity increases, due to presence of ions in voids.	Stability of crystal decreases and the electrical conductivity increases due to the formation of voids.

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Question 24 Marks

What is the effect of pressure and temperature on crystal structure?

Answer

Effect of Pressure on Crystal Structure:
Increase of pressure increases the coordination number during crystalization e.g. by applying high pressure, the NaCl crystal structure having 6 : 6 coordination changes to CsCl crystal structure having coordination number 8 : 8.
$\underset{(6: 6 \text { Coordination })}{ NaCl \text { type structure }} \xrightarrow{\text { High Temperature }} \underset{(8: 8 \text { Coordination })}{ CsCl \text { type structure }}$
Effect of Temperature on Crystal Structure :
Increase of temperature, decreases the co-ordination number e.g., on heating to 760 K CsCl structure having 8:8 coordination number changes to NaCl structure having coordination number 6 : 6.
$\underset{(8: 8 \text { Coordination })}{CsCl \text { type structure }} \xrightarrow{\text { High Temperature }} \underset{(6: 6 \text { Coordination })}{ NaCl \text { type structure }}$

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