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

Explanation

Transition metals are large in size and contain lots of interstitial sites. Transition elements can trap atoms of other elements that have small atomic size), such as H, C, N, in the interstitial sites of their crystal lattices. The resulting compounds are called interstitial compounds.
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.