Hi

So what are interstitials in a crystal? So answer it simply they are referred to atoms and ions which occupy the empty space or void present in the crystal. This sometimes leads to defects called interstitial defects. It is a crystallographic defect arising when a normally unoccupied site or vacant side gets occupied by atom of same or of a different type.

Interstitials can be produced by bombardment of a crystal by elementary elements which have energy above the displacement threshold of that crystal and can be present in small amounts in thermodynamics equilibrium.

Hope this helps