An interstitial defect is a type of point crystallographic defect where an atom of the same or of a different type, occupies a normally unoccupied site in the crystal structure. When the atom is of the same type as those already present they are known as a self-interstitial defect. Alternatively, small atoms in some crystals may occupy interstitial sites, such as hydrogen in palladium. Interstitials can be produced by bombarding a crystal with elementary particles having energy above the displacement threshold for that crystal, but they may also exist in small concentrations in thermodynamic equilibrium. The presence of interstitial defects can modify the physical and chemical properties of a material.
Self-interstitial defects are interstitial defects which contain only atoms which are the same as those already present in the lattice.
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  interstitial structure, where two atoms are displaced in a positive and negative  direction from the lattice site. In body-centered cubic (bcc) 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 (typically some 10–20) 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.
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 (at equal distances) 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.
Interstitials modify the physical and chemical properties of materials.