interstitial atom

In chemistry, an interstitial atom refers to a small atom or ion that occupies a void or "hole" within the crystal lattice structure of a solid material. This occurs when atoms or ions of a nonmetal fill the spaces between larger metal atoms or ions in a crystal.1

This phenomenon is a type of interstitial solid solution, where foreign atoms or ions fit into the spaces between the primary atoms of the crystal structure. These interstitial spaces can be like channel-like cavities within certain crystals. This is distinct from substitutional solid solution, where one ion replaces another in its atomic site.2