What are interstitial compounds? Give their examples and characteristics.

(N/A) Interstitial compounds are those which are formed when small atoms like $H, C,$ or $N$ are trapped inside the crystal lattice of transition metals.
Examples: $TiC, Mn_4N, Fe_3H, VH_{0.56}, TiH_{1.7}$ etc.
These compounds are generally non-stoichiometric and are neither typically ionic nor covalent. Their formulas do not correspond to any normal oxidation state of the metal.
Physical and chemical characteristics of interstitial compounds:
$(i)$ They have high melting points,higher than those of pure metals.
$(ii)$ They are very hard. Some borides approach diamond in hardness.
$(iii)$ They retain metallic conductivity.
$(iv)$ They are chemically inert.