an interstitial impurity

An interstitial impurity is a foreign atom that does not take over a lattice site but instead tucks itself into one of the small gaps between the host atoms. Picture oranges packed in a crate: the fruit sit in a regular array, but between them are little empty pockets. Now drop a handful of peas into those pockets — the peas are small enough to fit in the gaps without pushing the oranges out of place. Those peas are the interstitial impurities; the oranges are the host crystal.

The gaps in a crystal (interstitial sites) come in standard shapes and sizes — the octahedral holes and tetrahedral holes between the host atoms — and they are small, so only genuinely tiny foreign atoms can occupy them. In practice the interstitial-formers are the light elements: carbon, nitrogen, hydrogen, oxygen, boron. Carbon in iron is the classic case: a carbon atom (radius about 0.077 nm) sits in the larger octahedral holes of iron, still a squeeze that strains the surrounding lattice but small enough to fit. A big substitutional-sized atom simply could not — it would need a whole site of its own.

Interstitial impurities punch far above their weight because even in tiny amounts they distort the lattice and pin the defects that carry deformation. The strain field around each interstitial carbon atom is what makes steel hard: it obstructs the gliding dislocations that would otherwise let iron slip easily. Control of a fraction of a percent of interstitial carbon and nitrogen is, quite literally, the difference between soft iron and hardened steel.