Figure 5: FCC unit cell (atoms omitted). Figure 6: FCC unit cell divided Figure 7: FCC Interstitial tetrahedral (atoms omitted). atoms (8). Compute the radius r of an impurity atom that just fits into a FCC tetrahedral site in terms of the atomic radius R of the host atom (without introducing lattice strain). Which interstitial site can host a larger atom, octahedral or tetrahedral