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 principallattice directions. For instance, in several common face-centered cubic metals such as copper, nickel and platinum, the ground state structure of the self-interstitial is the split interstitial structure, where two atoms are displaced in a positive and negative direction from the lattice site. In body-centered cubic iron the ground state interstitial structure is similarly a 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 crowdion interstitial, which can be understood as a long chain of atoms along the 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 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.
Impurity interstitials
Small impurity interstitial atoms are usually on true off-lattice sites between the lattice atoms. Such sites can be characterized by the symmetry of the interstitial atom position with respect to its nearest lattice atoms. For instance, an impurity atom I with 4 nearest lattice atom A neighbours in an fcc lattice is in a tetrahedral symmetry position, and thus can be called a tetrahedral interstitial. Large impurity interstitials can also be in split interstitial configurations together with a lattice atom, similar to those of the self-interstitial atom. lattice. The actual interstitial atom would ideally be in the middle of one of the polyhedra.
Interstitial carbon atoms have a crucial role for the properties and processing of steels, in particular carbon steels.
Impurity interstitials can be used e.g. for storage of hydrogen in metals.
The crystall lattice can expand with the concentration of impurity interstitials
The amorphization of semiconductors such as silicon during ion irradiation is often explained by the buildup of a high concentration of interstitials leading eventually to the collapse of the lattice as it becomes unstable.
Creation of large amounts of interstitials in a solid can lead to a significant energy buildup, which on release can even lead to severe accidents in certain old types of nuclear reactors. The high-energy states can be released by annealing.
At least in fcc lattice, interstitials have a large diaelastic softening effect on the material.
It has been proposed that interstitials are related to the onset of melting and the glass transition.
