Atoms ‘A’ involve in CCP arrangement. Atoms 'B' occupy half of tetrahedral voids and atoms ‘C’ occupy half of the octahedral voids. Formula of the compound is

Atoms A in CCP; Number of atoms of A = 8 x 18+ 6 x12=4Number of TV =8; Number of atoms of B = 12 x 8 = 4 ;Number of OV = 4 ;Number of atoms of C = 4×12=2Molecular formula =A2B2C