Interstitial Sites (FCC & BCC) 1 •In the spaces between the sites of the closest packed lattices (planes), there are a number of well defined interstitial positions: •The CCP (FCC) lattice in (a) has 4 octahedral, 6-coordinate sites per cell; one site is at the cell center [shown in (a) and the rest are at the midpoints of all the cell edges (12·1/4) one shown in (a)]. •There are also 8 tetrahedral, 4-coordinate sites per unit cell at the (±1/4, ±1/4, ±1/4) positions entirely within the cell; one shown in (a)]. Alternative view of CCP (FCC) structure in (a): •Although similar sites occur in the BCC lattice in (b), they do not possess ideal tetrahedral or octahedral symmetry.