Which diffusion mechanism involves an atom moving from a lattice site into a neighboring vacant lattice site?

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

Answer: (B)

Vacancy diffusion is when an atom sitting on a regular lattice site jumps into a neighboring empty lattice site, leaving behind a vacancy that has moved to the atom’s original position. This process requires a neighboring vacant site to accept the atom, so the vacancy effectively migrates through the crystal as atoms hop into adjacent vacancies. This is distinct from interstitial diffusion, where much smaller atoms move through the gaps between lattice atoms (interstitial sites) rather than moving from one lattice site to another. Surfaces and grain boundaries provide faster diffusion paths along interfaces, but the specific description in the question—that an atom moves from a lattice site into a neighboring vacant lattice site—maps directly to vacancy diffusion.