is an atom from the crystal that is crowded into an interstitial
site, a small void space that under ordinary circumstances is not occupied; In metals, a self-interstitial introduces
relatively large distortions in the surrounding lattice because the atom is substantially larger than the interstitial position in which it is situated; Consequently,
the formation of this defect is not highly probable, and it exists in very small concentrations,
which are significantly lower than for vacancies.