Interstitial Compound
A compound formed when small atoms occupy the spaces between metal atoms in a lattice.
Unit
Dimensionless
In depth
Hydrogen, carbon, nitrogen and boron are small enough to fit into the holes of a transition metal lattice without disrupting it. The metallic bonding survives, so conductivity is retained, but the trapped atoms block dislocation movement and make the material dramatically harder.
Examples in the real world
Steel as interstitial carbon in iron; tungsten carbide in cutting tools; titanium nitride coatings.