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 A2Bx the value of x is :

Number of effective A atoms = 8 corners × 1/8 per corner atom share = 1 atom / unit cellNumber of atoms on faces of a cube = 6 atomsIf one is missing from one face, number of atoms left = 5× ½ per face atom share =52per unit cellThe formula of the compound is AB5/2.On simplify we get, the formula A2B5.