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

AC^{2} = b^{2} = BC^{2} + AB^{2}

= a^{2}+a^{2} = 2a^{2}

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)πr^{2} and volume of the cube is a^{3} or (22r)^{3}

Therefore,

Packing efficiency = %

=

== 74%

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