text: Self-interstitial defects are interstitial defects which contain only atoms which are the same as those already present in the lattice.
  The structure of interstitial defects has been experimentally determined in some metals and semiconductors.
  Contrary to what one might intuitively expect, most self-interstitials in metals with a known structure have a split structure, in which two atoms share the same lattice site. Typically the center of mass of the two atoms is at the lattice site, and they are displaced symmetrically from it along one of the principal lattice directions. 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 body-centered cubic (bcc) iron the ground state interstitial structure is similarly a [110] split interstitial.
  These split interstitials are often called dumbbell interstitials, because plotting the two atoms forming the interstitial with two large spheres and a thick line joining them makes the structure resemble a dumbbell weight-lifting device.
  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, 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.
  In semiconductors the situation is more complex, since defects may be charged and different charge states may have different structures. For instance, in silicon, the interstitial may either have a split [110] structure or a tetrahedral truly interstitial one.
  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.