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

In a C.C,P array, number of tetrahedral voids is twice the number of atoms i.e.2 Z. T.H.V = 2× ZIn C.C.P arrangement lattice points are 4 and tetrahedral voids are 8.Y- forms-ccp/or fccX-occupy 2/3rd of Tetrahedral voidsNumber of Y-atoms=4 Number of X-atoms =23×8 =163 ( Number of tetrahedral voids are=8)Y : X =4 : 163             = 3 : 4 ⇒ Y3X4 OR X4Y3