Why can interstitial defects diffuse faster than vacancies?

• A. Interstitials are larger and can move through interstitial sites with more coordination
• B. Interstitials are immobile at room temperature
• C. Vacancies diffuse faster than interstitials
• D. Interstitials are smaller and can move through interstitial sites with less coordination

Answer: (D)

Diffusion via defects hinges on how much the surrounding lattice must be disturbed for a hop to occur. Interstitials are small atoms that sit in gaps between the regular lattice atoms, so they can hop from one interstitial site to another with only minor lattice distortion. This means a lower energy barrier and less disruption of neighboring bonds, especially since they don’t have to shove a whole atom into a neighboring site. In contrast, vacancy diffusion requires a lattice atom to move into a neighboring empty spot, which involves breaking and re-forming several bonds and greater rearrangement of atoms. Because the energy hurdle is usually smaller for interstitial hops, interstitial diffusion is faster. The idea that interstitials are smaller and move through interstitial sites with less coordination captures this, explaining why they diffuse more rapidly.