For hexagonal close packed ( hcp ) 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×Octahedral‌voids ∴C=
2
3
×(2×6)=8 Hence, C:A=8:6=4:3 Therefore, the formula of the crystal is C4A3.