Answer :

(a) The Stoichiometric defect shown by NaCl is Schottky defect and this is due to the comparable (almost similar) sizes of Na+ and Cl- ions.

(b) Let the unit cell edge length be ‘a’ and face diagonal AC = b.


AC2 = b2 = BC2 + AB2

= a2+a2 = 2a2

or b = 2a

If r is the radius of the sphere, we find

b = 4r = 2a

or a = = 22r (we can also write, r= )

We know, that each unit cell in FCC structure, has effectively 4 spheres. Total volume of four spheres is equal to 4×(4/3)πr2 and volume of the cube is a3 or (22r)3


Packing efficiency = %


== 74%

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