In a face centered cubic lattice, atoms A are at the corner points and atoms B at the face centered points. If atom B is missing from one of the face centered points, the formula of the ionic compound is:

If in a face centered cubic lattice, atoms A are at the corner points.Therefore, the contribution of A = 8 × 1/8 = 1If B is at the face centered points, then contribution of B = 6 × 1/2 = 3If one B is missing from one face, then its contribution decreases to 5/2per lattice, then the formula will be A2B5.