(a) What type of Stoichiometric defect is shown by NaCl and why?

(b) Calculate the efficiency of packing in case of a metal crystal for FCC unit cell.

(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%