1.

(a) Calculate number of particles in Face Centred Cubic (FCC) structure. (b) Write any two differences between Frenkel and Schottky defects.

Answer»

Solution :(a)Let a be the edge length of the cube
‘A’ : be the face diagonal
‘R’ : is the radius of the sphere b = 4r
• In A ABC
`b^(2) = a^(2) + a^(2)`
`b^(2) = 2a^(2)`
`b = a sqrt(2) xx a`
`4r =sqrt(2) xx a` ` a =4/(sqrt(2)) implies a =(2A)//(2t)`
• Volume of the cube = `a^(3) = ^2-j2r j`
• Each unit cell of CCP structure contain 4 spheres.
• Volume of the four spheres = 4 x — it `4/8 r^(3)`
Packing efficiency =`("Volume of 4 ATOMS in the unit cel"L)/(T"otal volume of the unit cel"l)`
`=(Z4x^(4)-NR^(3))/(2V2r) xx100`
Packing efficiency = 74%.
(b) Ionic solids showing Schottky defect -NaCl, KCl, CsCl, AgBr.
Ionic solids showing Frenkel defect – ZnS, AgCl, AgBrAgl.


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