Interstitials are all atoms sitting not on their regular place, but between other atoms. The picture shows the simple case of a self interstitial atom in an elemental fcc crystal.
If the crystal is viewed as periodic arrangement of hard spheres, interstitials sit in the interstices between the spheres. For the most prominent simple crystals there are two kinds of interstices: Octahedra- and Tetrahedra interstices or gaps.
There are two basic kinds of interstitials: Intrinsic and extrinsic interstitials:
Intrinsic interstitials are interstitials atoms of the same kind as the atoms of the crystal "self-interstitials"). They are practically non existent in elemental crystals (i.e. in all metals) with the big exception of Si, where intrinsic interstitials play an important role in diffusion and microdefect formation.
Extrinsic interstitials are interstitial atoms of a foreign (extrinsic) type, e.g. C in Fe or O in Si. They may diffuse directly through the lattice (i.e. without the help of vacancies) and play an important role in many technically relevant materials.

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go to 1.3.3 The larger View and Complications

go to Vacancy

go to Octahedral sites

go to Tetrahedral Sites

© H. Föll (Defects - Script)