Formation of Interstitial Compounds

An interstitial compound (sometimes also called an interstitial alloy) is a compound formed when small atoms -- typically hydrogen, boron, carbon or nitrogen, all considerably smaller than a typical metal atom -- become trapped within the small interstitial holes (gaps) present between the regularly packed atoms of a metal's crystal lattice, rather than substituting directly for a metal atom at a normal lattice site (contrast this with the substitutional alloys of section 4.3.8). Interstitial compounds are usually non-stoichiometric -- that is, they do not obey simple whole-number formula ratios the way an ordinary ionic or molecular compound does, because the exact number of small atoms that can be accommodated in the interstitial holes can vary somewhat depending on preparation conditions.

Transition metals form a substantial number of such interstitial compounds; representative textbook examples include titanium carbide (TiC), a non-stoichiometric zirconium hydride written as ZrH₁.₉₂ (the non-integer subscript itself a direct signature of non-stoichiometry), and manganese nitride, Mn₄N. The small interstitial atoms trapped within the host metal lattice confer several distinctive new properties on the resulting material, properties that neither the pure parent metal nor the pure interstitial element possesses on its own:

1. Interstitial compounds are characteristically hard, and yet they retain good electrical and thermal conductivity -- a combination not typically found together in non-metallic hard materials, but readily explained here because the underlying metallic lattice (and its delocalised conduction electrons) is still substantially intact.
2. They typically have melting points that are even HIGHER than those of the corresponding pure parent metal, since the trapped interstitial atoms add extra bonding interactions that further reinforce the existing metallic lattice. …