Frenkel defect is a dislocation defect as smaller ions (usually cations) are dislocated from normal sites to interstitial sites. Frenkel defect is shown by compounds having large difference in the size of cations and anions hence, alkali metal halides do not show Frenkel defect. Also, Schottky defect decreases the density of crystal while Frenkel defect has no effect on the density of crystal.

The distance between the body centered atom and one corner atom is $${{\sqrt 3 } \over 2}a$$.

Total number of carbon atoms per unit cell

= $$8 \times {1 \over 8} + 6 \times {1 \over 2} + 4$$ = 8

for fcc Z = 4.

d = $${{ZM} \over {{N_A}{a^3}}}$$

$$ \Rightarrow $$ M = $${{d{N_A}{a^3}} \over Z}$$

= $${{2.72 \times 6.023 \times {{10}^{23}} \times {{\left( {404 \times {{10}^{ - 10}}} \right)}^3}} \over 4}$$

= 27 g mol^–1