Interstitial sites are the spaces between atoms and are far smaller than vacancies, which are essentially the absence of an atom form the structure. Interstitial diffusion occurs when molecules move through the interstitial spaces of the substance, while vacancy diffusion is when the molecules move into the vacancies.
Because the interstitial spaces are far more numerous than the vacant spaces, and the bonding to the surrounding atoms in interstitial spaces is weaker, interstitial diffusion is faster than vacancy diffusion.
