Formation of Interstitial Compounds

What an interstitial compound is

Interstitial compounds form when small atoms — such as hydrogen, carbon, or nitrogen — become trapped inside the gaps (interstices) of a metal's crystal lattice. Because the atoms sit in the spaces between the metal atoms rather than replacing them or bonding to them in a conventional way, these compounds are usually non-stoichiometric, and are neither typically ionic nor typically covalent in character. Representative examples include:

Notice that the formulas quoted — including the fractional subscripts on hydrogen in $\mathrm{VH_{0.56}}$ and $\mathrm{TiH_{1.7}}$ — do not correspond to any normal oxidation state of the metal, which is itself a signature of this non-stoichiometric, interstitial character.

Characteristic properties

Because of how they are composed — small foreign atoms lodged within an otherwise intact metal lattice — interstitial compounds share a distinctive set of physical and chemical characteristics:

They have high melting points, higher than those of the pure metal itself. …