Q.What are interstitial compounds?

Interstitial compounds (small non-metal atoms trapped in the voids of a transition-metal lattice) are extremely hard, high-melting, chemically unreactive, and retain the metallic conductivity and lustre of the parent metal.

Transition metals possess vacant interstitial spaces (voids) between the atoms of their close-packed crystal lattices. When small atoms of non-metals such as hydrogen, carbon, nitrogen or boron are trapped inside these voids (either during formation of the metal or by later diffusion into it), non-stoichiometric or near-stoichiometric compounds called interstitial compounds are formed, e.g. $TiC$, $Fe_3H$, $Mn_4N$, $VH_{0.56}$, $ZrH_{1.92}$.

Properties:

1. High melting points: interstitial compounds usually have melting points even higher than that of the pure parent metal, because the interstitial atoms strengthen the metallic lattice.
2. Extreme hardness: they are very hard, in some cases (e.g. tungsten carbide, $WC$) approaching the hardness of diamond, making them useful for cutting tools and abrasives.
3. Retain metallic conductivity: since the metallic bonding/delocalised electron structure of the parent lattice is largely preserved, these compounds continue to conduct heat and electricity like metals. …