In a compound XY, which crystallizes in cubic structure.  Y occupies the corners and X the centre of the cube.  If one Y^- from corner is replaced by a Z^-, what is the simplest formula of resulting compound?

Dear Student

In Cubic Structure, there are 8 corners, 6 faces and 12 edges
Atom contribution at corner = 1/8 and at body centre = 1
Since at one corner Y is replaced by Z
Therefore,  no. of Y atoms per unit cell = 7* 1/8 = 7/8
                   no. of Z atoms per unit cell = 1* 1/8 = 1/8
                   no. of X atoms per unit cell = 1* 1 = 1

$Thus, formula = X{Y}_{\frac{7}{8}}{Z}_{\frac{1}{8}}=X_8{Y}_{7}Z$