In a crystal lattice, anions $A$ forms hcp array, $2 / 3$ of the tetrahedral voids are occupied by cations $C$. What is the formula of crystal?

For hexagonal close packed ( $h c p)$ crystal, there are 6 atoms per unit cell. That means anion $A=$ number of octahedral voids $=$ number of atoms per unit cell $=6$. We also know that, tetrahedral voids $=2 \times$ octahedral voids . $\therefore \quad C=\frac{2}{3} \times(2 \times 6)=8$ Hence, $C: A=8: 6=4: 3$ Therefore, the formula of the crystal is $C_4 A_3$.