Self-interstitial defects in metals are generally much less probable than vacancies because the interstitial site is smaller than the atom.

• A. More common.
• B. About as likely as vacancies.
• C. Much less probable due to size mismatch.
• D. Only occur at high temperature.

Answer: (C)

The important idea here is defect formation energy and how size mismatch affects stability. In a metal lattice, making a self-interstitial means forcing an extra atom into a normally empty interstitial site, which severely distorts the surrounding atoms and creates a large strain. This large distortion costs a lot of energy, giving a high formation energy for interstitials. Vacancies, on the other hand, simply remove an atom from its lattice site and the lattice can relax with less overall disruption, so the formation energy is typically lower. Since the equilibrium concentration of defects scales like exp(-formation_energy/kT), interstitials are far less probable than vacancies at a given temperature. The statement that the interstitial site is smaller than the atom explains why the energy cost is so high, making self-interstitials rare unless additional energy is supplied (as in irradiation, which can create Frenkel pairs). That’s why the correct view is that self-interstitials are much less probable due to the size mismatch.