Question

What are interstitial compounds? Why are such compounds well known for transition metals?

Answer 1

Interstitial compounds are those which are formed when small atoms like, $H,N$ or $C$ are trapped inside the crystal lattices of metals. They are usually non-stoichiometric and are neither typically ionic or covalent.

Interstitial compounds are well known for transition compounds due to its closed crystalline structure with voids in them. The atomic size of transition metals are very large hence have large voids to occupy these small atoms.