In a compound, atoms of element Y form ccp lattice and those of element X occupy 2/3rd of tetrahedral voids. The formula of the compound will be

Y occupies CCP lattice hence the effective number of Y = 4 (effective no of atoms in CCP is 4) X occupies 2/3 of tetrahedral voids. Number of tetrahedral voids generated in any unit cell is = 2Z (twice of an effective number of atoms in unit cell) 2 4 8 x Since X occupies 2/3 of tetrahedral voids, so Effective number of X= 2 16 ×8= 3 3 The formulae will be 4 3 X Y Hence the correct answer is option (a)