Which type of crystal defect shown by following fig.? $Na ^{+} Cl ^{-} Na ^{+} Cl ^{-} Na ^{+} Cl ^{-}$ $Cl ^{-} Cl ^{-} Na ^{+} Na ^{+}$ $Na ^{+} Cl ^{-} Cl ^{-} Na ^{+} Cl ^{-}$ $Cl ^{-} Na ^{+} Cl ^{-} Na ^{+} Na ^{+}$
A compound formed by X and Y elements, crystallizes in cubic structure, in which X atoms are at the corner and Y atoms are at the face-centre of the cube. The formula of the compound is: