In a face centred cubic lattice, atom A occupies the corner positions and atom B occupies face centre positions, if one atom of B is missing from one of the face centred points, the formula of the compounds is A₂B_x the value of x is :

Number of effective A atoms 

= 8 corners × 1/8 per corner atom share 

= 1 atom / unit cell

Number of atoms on faces of a cube = 6 atoms

If one is missing from one face, number of atoms left = 5

× ½ per face atom share =$\frac{5}{2}$per unit cell

The formula of the compound is AB_5/2.

On simplify we get, the formula A₂B₅.