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

Question

Philoid · Accepted Answer

Interstitial compounds are those compounds in which small atomic size elements like H, C, N occupy the interstitial sites of the crystal lattice of the Transition metal due to their large size.

Example, in the diagram given below:

Here, Green Balls form the Crystal lattice of transition metal and Blue balls represent elements like H/C/N which occupy the interstitial sites.