Answer and Explanation:
Interstitial diffusion allows the purity atom like (C, O, H) to occupy the interstitial sites in the atom which are abundantly present as a result of weaker bond formation with the surrounding atom.
The interstitial diffusion of iron in Body Centered Cubic (BCC) crystal lattice is faster than Face Centered Cubic (FCC) and the reason being that BCC structure is more open with more interstitial spaces than FCC structure.
Also the packing fraction of BCC lattice is less than that of FCC lattice.
For BCC is 0.68 whereas for FCC, it is 0.74