Write a note on interstitial compounds.

(N/A) Interstitial compounds are those which are formed when small atoms like $B$,$H$,$C$,or $N$ are trapped inside the crystal lattice of metals.
The transition elements form interstitial compounds with these elements. The $B$,$H$,$C$,and $N$ occupy the vacant spaces (interstitial sites) in the crystal lattice,and as a result,the compounds formed are extremely hard.
They are classified as borides,hydrides,carbides,or nitrides depending on the nature of atoms trapped in the vacant sites. They are usually non-stoichiometric,neither typically covalent nor ionic. Examples include $TiC$,$Mn_4N$,$Fe_3H$,$VH_{0.58}$,and $TiH_{1.7}$.
The important physical and chemical characteristics of these compounds are:
$(i)$ High melting points,even higher than those of pure metals.
$(ii)$ They are extremely hard. Some borides approach diamond in hardness.
$(iii)$ They retain metallic conductivity.
$(iv)$ They are chemically inert.