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.



In ∆ABC


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


Therefore,


Packing efficiency = %


=


== 74%


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