Which description characterizes a self-interstitial defect?

• A. A missing atom at a lattice site.
• B. An atom occupying an interstitial site that causes little distortion.
• C. A substitutional impurity.
• D. An atom occupying an interstitial site that causes large distortions.

Answer: (D)

Self-interstitial defects occur when an extra atom sits in a position between regular lattice points, pushing nearby atoms and creating noticeable lattice distortion. This happens because the interstitial site isn’t meant to host an atom of the same size as those in the lattice, so the surrounding structure is strained. The description that matches this behavior is an atom occupying an interstitial site that causes large distortions. The other possibilities refer to different defect types: a missing atom at a lattice site is a vacancy, a substitutional impurity is a foreign atom replacing a lattice atom, and an interstitial atom that causes little distortion would not typically describe how an extra atom disrupts the lattice.