What is a vacancy defect?

• A. An empty lattice site where an atom should be
• B. A foreign atom substituting for a host atom
• C. An extra atom occupying an interstitial site
• D. A line defect representing a misfit at the lattice edge

Answer: (A)

A vacancy defect is an empty lattice site where an atom would normally be located. It’s a basic point defect in a crystal, created when an atom is missing from its regular position. Vacancies enable diffusion because nearby atoms can hop into the empty site, and their presence affects properties like diffusion rates, strength, and electrical behavior. The number of vacancies at a given temperature depends on the formation energy of creating the defect and follows an Arrhenius-type relation.

If a foreign atom sits on a lattice site, that’s a substitutional defect, not a vacancy. If an extra atom resides in an interstitial space (a site between lattice points), that’s an interstitial defect. A line defect involves a dislocation along a line, describing a misfit at the lattice edge rather than a missing atom.