Interstitial defect

The idea of interstitial compounds was started in the late 1930s and they are often called Hagg phases after Hägg.

In both of these very similar lattices there are two sorts of interstice, or hole: It was suggested by early workers that: These were not viewed as compounds, but rather as solutions, of say carbon, in the metal lattice, with a limiting upper “concentration” of the smaller atom that was determined by the number of interstices available.

For instance, in several common face-centered cubic (fcc) metals such as copper, nickel and platinum, the ground state structure of the self-interstitial is the split [100] interstitial structure, where two atoms are displaced in a positive and negative [100] direction from the lattice site.

In other bcc metals than iron, the ground state structure is believed based on recent density-functional theory calculations to be the [111] crowdion interstitial,[4] which can be understood as a long chain (typically some 10–20) of atoms along the [111] lattice direction, compressed compared to the perfect lattice such that the chain contains one extra atom.

[5] Carbon, notably in graphite and diamond, has a number of interesting self-interstitials - recently discovered using Local-density approximation-calculations is the "spiro-interestitial" in graphite, named after spiropentane, as the interstitial carbon atom is situated between two basal planes and bonded in a geometry similar to spiropentane.