Interstitial defects ( 2 ) are point defects that deal with atoms that fill otherwise unoccupied sites in a crystal lattice. These defects can be self-interstitial, wherein an atom of the original material is situated in the wrong position, or they can be impurities, wherein an atom of another element is inserted into the lattice.
Self-interstitials are often split interstitials, or dumbbell interstitials, wherein two atoms occupy the same lattice site, each of them slightly shifted from exact position. The upper right image above depicts a split interstitial.
If an interstitial of a foreign atom is introduced, it is usually called an impurity, but can also be classified as a solute, if the introduction of the interstitial was intentional. Interstitial alloys/compounds are fairly common, and are types of solid solutions which include low carbon steels. Creating interstitials is also a method of solution hardening a material.
Image sources: ( 1 ) ( 3 )
