What are interstitial compounds? What are their main characteristics?

In the crystal lattices of transition metals-small atoms like C, H, B, N etc. are trapped non-stoichiometrically. Such compounds are called interstitial compounds, e.g., TiC, Mn₄N, Fe₃H, TiH₂ etc. Their main characteristics are:
(i)    They have high melting points than their metals.
(ii)    They become much harder (e.g., borides).
(iii)    Metallic conductivity is retained.
(iv)    They become chemically inert.
(v)    They become less malleable and ductile.