Question

In a binary compound, atoms of element $A$ form a hcp structure and those of elements $M$ occupy $2/3$ of the tetrahedral voids of the hcp structure. Find the formula of the compound.

• A. $M_4{A}_3$
• B. $M{A}_3$
• C. $M_{4}A$
• D. $M_2{A}_3$

Answer 1

The correct option is A $M_4{A}_3$

Effective number of atoms in hcp unit cell $=6$
So, $A=6$
Total number of tetrahedral voids in hcp unit cell $=12$
$\therefore M=\frac{2}{3}\times 12=8$
Thus, the formula of the binary compound would be $A_6{M}_8\text{or}{A}_3{M}_4\text{or}{M}_4{A}_3$

Hence, option (b) is the correct answer.