Which diffusion mechanism is typically responsible for diffusion of very small atoms in solids?

• A. Interstitial diffusion
• B. Vacancy diffusion
• C. Grain boundary diffusion
• D. Surface diffusion

Answer: (A)

Interstitial diffusion is the mechanism at play when very small atoms move through solids. These solute atoms are small enough to fit into the spaces between the regular lattice atoms, called interstitial sites. They diffuse by hopping from one interstitial site to another, without displacing the surrounding host atoms. This path requires relatively low energy barriers because the lattice isn’t being rearranged as a whole, so diffusion rates for these tiny atoms are typically high. In contrast, vacancy diffusion involves moving into lattice vacancies and tends to be slower for very small solutes, since it relies on exchanging positions with host atoms and can require more lattice distortion. Surface and grain boundary diffusion occur along surfaces or defect regions and describe different geometries, not the bulk diffusion of tiny solutes in the crystal interior.