Total effective number of atoms in hcp unit lattice = Number of octahedral voids in hcp=6∴ Number of tetrahedral voids (TV) in hcp =2× Number of atoms in hcp lattice =2×6=12As, formula of the lattice is A2​B3​. Suppose, (31​×TV) (hcp)⇒31​×12 6⇒32​ 1⇒2 3So, A=31​ tetrahedral voids, B= hcp lattice