Suppose the number of atoms Y in the hcp lattice = n As the number of tetrahedral voids is double the number of atoms in close packing, the number of tetrahedral voids = 2n As atoms X occupy 2∕3rd of the tetrahedral voids, the number of atoms X in the lattice
2
3
×2n=
4n
3
∴ Ratio of X:Y=
4n
3
;n=
4
3
:1=4:3 Hence, the formula of the compound is X4Y3.