Which statement best describes a self-interstitial defect in metals?

• A. It has negligible lattice distortion.
• B. It is very common.
• C. It causes relatively large distortions and occurs in very small concentrations.
• D. It occurs only in ionic crystals.

Answer: (C)

The key idea is how a self-interstitial affects the metal lattice. A self-interstitial is when an extra atom sits in a normally empty interstitial site (a space between lattice points). That extra atom has to fit into a space that’s too small for it, so it pushes nearby atoms away and creates a noticeable local distortion. Because placing an atom into an interstitial site requires a high amount of energy, the equilibrium concentration of self-interstitials is very small, even though the distortion around that site is large. This combination—large local distortion but very low concentration—best describes the defect in metals. The other statements don’t fit: the distortion isn’t negligible, interstitials are not very common, and self-interstitials can occur in metals (not only in ionic crystals).